[m-rev.] for post-commit review: convert ssdebug to use var_tables

[Julien Fischer]

On Tue, 19 Apr 2022, Zoltan Somogyi wrote:

> Convert the ssdebug transform to use var_tables.
> 
> compiler/ssdebug.m:
>     As above.
> 
> compiler/polymorphism_type_info.m:
>     Move the comment text describing the structure of type_infos
>     from the start of polymorphism.m to here.
>
>     Provide a version of a predicate that takes a var_table instead of
>     a varset and a vartypes structure.
>
>     Provide versions of predicates used by ssdebug.m that take and update
>     just the data they need, rather than some larger data structure.
>
>     Replace some uses of booleans with bespoke types. Explain the complicated
>     logic behind their use.
>
>     Break up a too-large predicate.
>
>     Rename a predicate to avoid an ambiguity.
> 
> compiler/polymorphism.m:
>     Delete the comment text moved to polymorphism_type_info.m.
>
>     Conform to the changes above.

The diff is fine, however there are comments containing pointers to
polymorphism.m in the runtime that probably need to be updated to point
to polymorphism_type_info.m (e.g. in runtime/mercury_type_desc.h, 
runtime/mercury_type_info.h).

Julien.