Improve match subject inference

[cdce8p]

At the moment mypy only infers the type of variables captures inside a match statement. This PR adds additional type information to the match subject itself, similar to isinstance calls.

class A:
    a: str | None

m: object
match m:
    case A(a="Hello"):
        reveal_type(m.a)  # str | None -> Literal['Hello']

Fixes #14911

[github-actions]

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