Fix exhaustive match after unreachable cases

[puneetdixit200]

Fixes #21517

Summary

• keep match exhaustiveness from being updated by cases that are already unreachable
• still type-check unreachable cases so existing diagnostics are preserved
• add a regression for an exhaustive A | None match followed by an unreachable B() case

Tests

• .venv\Scripts\python -m pytest -n0 mypy/test/testcheck.py::TypeCheckSuite::check-python310.test
• .venv\Scripts\python -m pytest -n0 mypy/test/testcheck.py::TypeCheckSuite::check-unreachable-code.test
• .venv\Scripts\python -m mypy --enable-error-code exhaustive-match -c "class A: pass\nclass B: pass\nx: A | None\nmatch x:\n case None:\n pass\n case A() as a:\n pass\n case B():\n pass"
• .venv\Scripts\python -m pre_commit run --files mypy/checker.py test-data/unit/check-python310.test
• git diff --check

LLM assistance was used while iterating on the patch and tests. I reviewed the change and can explain it.

[github-actions]

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