[m-rev.] for review: use var_dbs in polymorphism

[Julien Fischer]

On Fri, 29 Apr 2022, Zoltan Somogyi wrote:

> Make the polymorphism pass use var_dbs ...
> 
> ... to allow its services to be used by passes that use var_tables
> as well as passes that still use varsets and vartypes.
> 
> compiler/polymorphism_info.m:
>     Store a var_db in the polymorphism_info structure instead of
>     a varset/vartypes pair. Make the same chane in the var_maps type,

s/chane/change/

>     which stores snapshots of parts of polymorphism_info.
> 
> compiler/polymorphism_type_info.m:
> compiler/polymorphism_lambda.m:
>     For each predicate that operated on varset/vartype pairs,
>     add a version that operates on var_dbs.
>
>     An exception is init_type_info_var, for which we require its caller
>     to pass it the variable to initialize, allocating that variable
>     if necessary.
>
>     Add some explanatory comments.

That's fine.

Julien.