Fix 13777 - refine check for caching companion in sum mirror

[bishabosha]

This also exposes a check to see if a file is a "lateCompile" in the current run, as a late-compiled symbol's type info is derived from source - aka a companion that is late-compiled would not subtype Mirror.Sum.

Edit: the above is no longer a part of these changes

cache the mirror in the companion if:

• the class is not scala 2 defined
  & the class has a companion
  & the class was either:
  • defined in source
  • from tasty and its companion is a subtype of Mirror.Sum

fixes #13777

[bishabosha]

I have no idea what's happening with the fs2 test in the community build - there are some comparison tests that are producing unexpected results - does not seem related to this

Edit- was fine after re-running CI

[bishabosha]

@smarter I have implemented the simpler version of a from-source check

[smarter]

It'd be good to explain why for things which are not from source we need the more precise check (i.e., the difference in code generation between scala 3.0 and 3.1).

[dwijnand]

It's more about the other way round: ideally we can just check is subclass (which is only true in scala 3.1+). But we call this check before that parent is added, for code in compilation, so have to handle that specially.

Feels like the two new ones would subsume !self.is(Scala2x), right? (minor)

[bishabosha]

I have clarified the comment to explain why we check if its from source

[dwijnand]

Thanks, Jamie, for picking this up. Good to see it didn't spin out from the fix we had theorycrafted. Good to go, for me. I would hope this could make the 3.1.1 patch release.