Use {err,warn,info}_spec as subtypes of diag_spec ...

... to replace pretty much all uses of diag_spec in the
initial part of the compilation process.

The point of this is that up to about mode analysis, we use
the presence or absence of errors of particular kinds to make
decisions about whether or not to stop the compilation process.
We do this to allow later stages to be able to omit having to handle
missing information that earlier passes have already reported.

compiler/error_spec.m:
    Add spec_severity_{err,warn,info} as subtypes of spec_severity.

    Use those to add {err,warn,info}_spec as subtypes of diag_spec.

    Rename the "diag_spec" function_symbol of the diag_spec type
    to "gen_spec". The old name is simply inappropriate for the
    new subtypes.

    Add versions of utility predicates that work on the new subtypes.

    Add a workaround for limitation of coerce operations.

compiler/maybe_error.m:
    Make the maybeN types more strict by default by replacing
    one_or_more(diag_spec) as the argument type of the errorN
    function symbols with one_or_more(err_spec). This prevents bugs
    (of which this diff found some) where

    - a compiler component reports an error, but
    - the attached diag_specs contain no actual errors.

    Ruling out such bugs is a secondary objective of this diff.

    A bunch of places that report errors also report warnings.
    Provide mechanisms for attaching warnings both to errorN
    function symbols (in the presence of errors) and to okN
    function symbols (in their absence).

    Provide versions of existing operations that work on values
    of the new types.

    Delete existing operations for maybe[56] types where
    those operations were not used anywhere.

compiler/error_util.m:
compiler/write_error_spec.m:
    Add versions of utility predicates that work on the new subtypes.

compiler/add_class.m:
compiler/add_clause.m:
compiler/add_foreign_enum.m:
compiler/add_foreign_proc.m:
compiler/add_mode.m:
compiler/add_mutable_aux_preds.m:
compiler/add_pragma_decl.m:
compiler/add_pragma_gen.m:
compiler/add_pragma_impl.m:
compiler/add_pragma_tabling.m:
compiler/add_pragma_type_spec.m:
compiler/add_pragma_type_spec_constr.m:
compiler/add_pred.m:
compiler/add_type.m:
compiler/check_field_access_functions.m:
compiler/check_import_accessibility.m:
compiler/check_libgrades.m:
compiler/check_module_interface.m:
compiler/check_options.m:
compiler/check_promise.m:
compiler/check_type_inst_mode_defns.m:
compiler/check_typeclass.m:
compiler/clause_to_proc.m:
compiler/color_schemes.m:
compiler/convert_import_use.m:
compiler/convert_include.m:
compiler/convert_parse_tree.m:
compiler/d_file_deps.m:
compiler/deps_map.m:
compiler/du_type_layout.m:
compiler/hlds_clauses.m:
compiler/hlds_data.m:
compiler/hlds_pred.m:
compiler/instance_method_clauses.m:
compiler/make_hlds_error.m:
compiler/make_hlds_passes.m:
compiler/make_hlds_warn.m:
compiler/mercury_compile_args.m:
compiler/mercury_compile_augment.m:
compiler/mercury_compile_front_end.m:
compiler/mercury_compile_main.m:
compiler/mercury_compile_make_hlds.m:
compiler/module_qual.mq_info.m:
compiler/module_qual.qual_errors.m:
compiler/module_qual.qualify_items.m:

compiler/options_file.m:
compiler/parse_class.m:
compiler/parse_dcg_goal.m:
compiler/parse_error.m:
compiler/parse_goal.m:
compiler/parse_goal_util.m:
compiler/parse_inst_mode_defn.m:
compiler/parse_module.m:
compiler/parse_mutable.m:
compiler/parse_pragma.m:
compiler/parse_pragma_foreign.m:
compiler/parse_pragma_tabling.m:
compiler/parse_sym_name.m:
compiler/parse_tree_out.m:
compiler/parse_tree_out_clause.m:
compiler/parse_type_defn.m:
compiler/parse_type_name.m:
compiler/parse_type_repn.m:
compiler/parse_types.m:
compiler/parse_util.m:
compiler/parse_vars.m:
compiler/prog_event.m:
compiler/prog_foreign_enum.m:
compiler/prog_item.m:
compiler/prog_parse_tree.m:
compiler/source_file_map.m:
compiler/split_parse_tree_src.m:
compiler/state_var.m:
compiler/superhomogeneous.m:
compiler/superhomogeneous_lambda.m:
    Return one (or more) of {err,warn,info}_specs instead of
    diag_specs. Perform the same replacements in data structures.
    (In most modules, diag_specs are replaced by *only* err_specs.)

    In modules where the diag_specs have all been replaced
    by the same subtype, keep using Specs as (part of) the names
    of the variables containing them. In modules that use
    more than one of the new subtypes, use ErrSpecs, WarnSpecs
    and/or InfoSpecs.

    Consistenly put err_specs before warn_specs, and both
    before info_specs.

    Change several predicates that used to take in,out pairs
    of list(diag_spec)s to return a single {err,warn}_spec,
    since this makes it easier to handle diagnostics of
    different severities separately.

    In a small handful of cases, replace code that created a diag_spec
    of dynamically-decided severity with two pieces of code that each
    create a diag_spec of a statically-decided severity, so that
    they can be added to different lists.

compiler/add_pragma_util.m:
    As above, and also stop exporting two functions that are needed
    only in this module, and move them to the end of the module,
    after all their callers.

compiler/grab_modules.m:
    As above, but record warnings as *warnings*, instead of
    as nonfatal errors, which is what we used to do.

compiler/parse_item.m:
    As above, but change the behavior when parsing a promise
    yields no error but at least one warning. We use to print
    only the warnings, and then exit with a nonzero exit status,
    which is strange. So now, in such situations, also print
    a message about warnings in promises BEING errors themselves.

compiler/accumulator.m:
compiler/common.m:
compiler/comp_unit_interface.m:
compiler/cse_detection.m:
compiler/decide_type_repn.m:
compiler/det_check_proc.m:
compiler/det_check_switch.m:
compiler/det_infer_goal.m:
compiler/equiv_type.m:
compiler/equiv_type_parse_tree.m:
compiler/error_sort.m:
compiler/fact_table_gen.m:
compiler/field_access.m:
compiler/file_util.m:
compiler/float_regs.m:
compiler/get_dependencies.m:
compiler/goal_expr_to_goal.m:
compiler/handle_options.m:
compiler/make.build.m:
compiler/make.get_module_dep_info.m:
compiler/make.library_install.m:
compiler/make.module_target.m:
compiler/make.program_target.m:
compiler/make.top_level.m:
compiler/make.track_flags.m:
compiler/make_hlds_separate_items.m:
compiler/mark_tail_calls.m:
compiler/mark_trace_goals.m:
compiler/mode_errors.m:
compiler/mode_info.m:
compiler/modes.m:
compiler/module_baggage.m:
compiler/module_qual.collect_mq_info.m:
compiler/module_qual.id_set.m:
compiler/old_type_constraints.m:
compiler/opt_deps_spec.m:
compiler/opt_format_call.m:
compiler/pd_util.m:
compiler/polymorphism_goal.m:
compiler/post_typecheck.m:
compiler/prog_mode.m:
compiler/purity.m:
compiler/qual_info.m:
compiler/read_modules.m:
compiler/recompilation.check.m:
compiler/recompilation.used_file.m:
compiler/simplify_goal.m:
compiler/simplify_goal_call.m:
compiler/stratify.m:
compiler/style_checks.m:
compiler/term_constr_errors.m:
compiler/term_errors.m:
compiler/typecheck.m:
compiler/typecheck_error_arg_vector.m:
compiler/typecheck_error_overload.m:
compiler/typecheck_error_undef.m:
compiler/typecheck_error_unify.m:
compiler/typecheck_error_wrong_type.m:
compiler/unused_imports.m:
compiler/warn_unread_modules.m:
compiler/write_deps_file.m:
    Conform to the changes above.

tests/invalid_options_file/undefined_var.err_exp:
    Delete the initial "mmc:" from the expected output,
    since that is a prefix on message about errors, and
    the output of this test contains only warnings.
    With the diff to mercury_compile_args.m above,
    the compiler now recognizes this.