GeezerVison

Fixes #136952.

The C analyzer preprocesses source files in C99 mode. With the headers used
in the reported configuration, static assertions expand into compatibility
declarations that the parser cannot handle. This causes analysis of
Python/gc_free_threading.c to fail when configured with --disable-gil.

This change:

• Uses C11 preprocessing to avoid the compatibility expansion.
• Handles _Static_assert and static_assert declarations at file scope
  and inside structs and unions, reusing the existing parenthesis-matching
  logic.
• Adds focused regression tests to verify that declarations following
  static assertions are still discovered.

The assertions in CPython's C code remain unchanged.

Testing

Validated with a locally built CPython 3.16 free-threading interpreter:

• Reproduced the reported analysis failure before the fix.
• Ran python -m test test_tools: 80 tests run, 1 skipped, successful.
• Ran check-c-globals.py on Python/gc.c,
  Python/gc_free_threading.c, and Objects/listobject.c:
  0 failures.
• Temporarily restored C99 preprocessing in memory and confirmed that
  the preprocessing regression test fails.
• git diff --check passes.

The full CPython test suite was not run.

AI assistance

GPT-6 Astra via OpenCode v2 assisted with investigating the issue,
implementing the changes, and preparing the tests and this PR description.

• Issue: The C analyzer seems to fail when --disable-gil is set #136952