Report irrefutable match patterns that make later cases unreachable - #21945

Fixes #21925.

CPython rejects at compile time a match statement whose non-final case is an unguarded capture or wildcard pattern, or that contains an or-pattern with an irrefutable alternative in a non-final position (SyntaxError: name capture 'y' makes remaining patterns unreachable / wildcard makes remaining patterns unreachable), since such patterns make the remaining cases unreachable. mypy currently checks such files clean — a file that cannot even be imported passes type checking.

What

Mirror CPython's irrefutability check (PEP 634) in the match statement checker:

• an unguarded capture or wildcard in a non-final case is reported (a guard or being the final case makes it fine);
• an irrefutable alternative in a non-final position of an or-pattern is reported, at any nesting depth (including inside sequence, mapping and class patterns);
• captures inside composite patterns ([x], Cls(x), {k: x}) stay allowed, matching CPython — only the positions above are inspected.

Differences from CPython:

• mypy reports every occurrence, while CPython stops at the first SyntaxError;
• this is a regular error (not a syntax error), so the file still type checks after the report.

Implementation notes

• The check runs before type-dependent narrowing, purely on the pattern AST, and is skipped on deferred passes to avoid duplicate reports.
• case y as z: is handled naturally: the parser represents y as an inner capture pattern, which the recursion inspects with the outer allow flag — matching CPython's own AST (MatchAs(pattern=MatchAs(name='y'), name='z')).
• Added sub_patterns() to mypy/patterns.py for enumerating composite sub-patterns.

Verification

• 10 new unit tests in check-python310.test covering the full rule matrix (capture/wildcard non-final, guard, final case, or-pattern positions, nested or-patterns, composite captures allowed, x as y, multiple violations).
• Full check suite: 8214 passed, 15 skipped, 7 xfailed — no regressions.
• mypy --config-file mypy_self_check.ini clean; ruff check and ruff format clean on touched files (modulo two pre-existing format drifts on master).