Fix Issue 23226 - ImportC: allow single-argument _Static_assert(expr)

[AayushMainali-Github]

Summary

• ImportC now accepts C23's single-argument _Static_assert(expr) in addition to the existing _Static_assert(expr, "msg") form.
• Message parsing is skipped unless a comma follows the constant-expression; a missing message builds StaticAssert with a null msgs array (same as D's static assert).
• Adds compilable/fail_compilation coverage and a changelog entry.

Fixes #23226

Test plan

• make -j$(nproc) (compiler + druntime rebuild)
• ./run.d compilable/test23226.c fail_compilation/test23226.c
• Spot-check existing ImportC _Static_assert tests still pass (failcstuff1.c, test24594.c)

[dkorpel]

Add the test cases to the existing ones instead of a new file

[github-actions]

DMD perf check

Metric	Base	PR	delta
compile hello.d (instr)	216.3 M	216.3 M	0.00%
compile hello.d -O (instr)	234.9 M	234.9 M	0.00%
dmd binary size (stripped)	6.86 MB	6.86 MB	0.00%
hello binary size	0.72 MB	0.72 MB	0.00%
peak RSS (compile hello.d)	46 MB	46 MB	-0.12%

[AayushMainali-Github]

Add the test cases to the existing ones instead of a new file

Moved the tests into testcstuff1.c / failcstuff3.c.

Tests Moved