Address false positive cases for return value codefix

[rbuckton]

In #26434 we added a code fix that can insert a return statement in places where it is missing, however the code fix has a step that is intended to verify that the resulting return expression will be correctly assignable. Unfortunately, this code fix attempts to type check a purely synthetic AST with no original pointer set, which results in the call to checker.getTypeAtLocation essentially returning any. This results in false-positives for code fixes that should not be valid.

This changes the code fix to instead leverage the checker.createSignature and checker.createAnonymousType functions to synthesize the expected type for comparison.

[Kingwl]

Thanks!

[weswigham]

@rbuckton you planning on merging this? 😄

[rbuckton]

It looks like this was updated and merged as part of #35282.