Add -fno-cxx-modules to clang's known -Xclang flags

[badumbatish]

Downstream Bazel's C++ toolchain passes -Xclang -fno-cxx-modules in its compiler sanity checks. Since -fno-cxx-modules wasn't in sccache's known flag list, the xclang argument parser in gcc.rs treated it as an UnknownFlag and bailed with CannotCache.

[sylvestre]

please add a test

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.32%. Comparing base (8825b4a) to head (71cfd42).
⚠️ Report is 19 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2717      +/-   ##
==========================================
- Coverage   74.41%   74.32%   -0.09%     
==========================================
  Files          70       70              
  Lines       39211    39374     +163     
==========================================
+ Hits        29177    29266      +89     
- Misses      10034    10108      +74     

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[codspeed-hq]

Merging this PR will improve performance by 4.55%

⚡ 1 improved benchmark
✅ 59 untouched benchmarks
⏩ 4 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	normalize_win_path_typical	1.6 µs	1.5 µs	+4.55%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing badumbatish:no_cxx_module (71cfd42) with main (8825b4a)}

Footnotes

1. 4 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩