Allow redefinitions for function arguments

[ilevkivskyi]

Fixes #19918

Implementation is straightforward, but logic behind is essentially a compromise. Both empty context and full type context for redefinition have pros and cons. We select full context first for function arguments because:

• It avoids false positives in at least some of the "covariant normalization" patterns. For example list[str | None] to list[str] in a branch will infer list[str | None] instead of list[str | None] | list[str]. Unconditional normalization like list[str | None] to list[str] still doesn't work, but it doesn't work for empty context either.
• Conceptually function arguments are in between annotated and annotated variables, in the sense that user doesn't really have an option to leave them unannotated if they want the redefinition semantics. So we allow redefinition but use type context behavior like for annotated variables.

cc @JukkaL

[ilevkivskyi]

As I suspected, for argument types, defaulting to full type context will be probably better (with a fallback to empty context in case of errors only).

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[ilevkivskyi]

@JukkaL To speed things up I am going to merge this soon. Please feel free to take a look later, but as I mentioned, after I refactored/unified the fallback logic into a separate helper method, the implementation is trivial (and we seem to agree on the desired semantics).