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:

graphql-core (https://github.com/graphql-python/graphql-core)
- version: 1.19.0+dev.19697af9051707b4db55bc2d5301436d872f452c
+ version: 1.19.0+dev.4255264d63f49d53ac0fa82fe2aee7d48872635d