[m-rev.] for post-commit review: clarify some used_modules code

[Peter Wang]

On Wed, 30 Mar 2022 02:18:44 +1100 "Zoltan Somogyi" <zoltan.somogyi at runbox.com> wrote:
> Clarity some code for computing used modules.
> 
> compiler/hlds_module.m:
>     Document the semantics of the used_module field, and describe a possible
>     bug.
> 
>     Give a predicate for recording modules as used a more descriptive name.
> 
> compiler/make_hlds_passes.m:
>     Conform to the change in hlds_module, and factor out both
>     common code and common comments.
> 
> compiler/prog_data_used_modules.m:
>     Move an invariant test on visibility out of a loop.

That's fine.

Peter