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]

Diff from mypy_primer, showing the effect of this PR on open source code:

pytest (https://github.com/pytest-dev/pytest)
+ src/_pytest/assertion/rewrite.py:1023: error: Unused "type: ignore" comment  [unused-ignore]