Codegen: stats and progress issues

[ysbaddaden]

The --stats and --progress params had a couple issues:

• codegen progress isn't updated when --threads=1 (always the case on Windows);
• only stats need to collect reused modules (progress doesn't).

Idea for a follow-up: the report fiber of forked codegen could set unit.reused_previous_compilation instead of collecting reused module names, then we wouldn't need to collect the module names into a reused array (we can trust units to be updated) and the #print_codegen_stats method would count & filter has needed.

[ysbaddaden]

@sdogruyol I assume you faced the issue?

[sdogruyol]

@sdogruyol I assume you faced the issue?

Yes, I recently experienced --stats issue on Windows

[ysbaddaden]

Merged with master to fix conflict with #14762.