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 72.92%. Comparing base (8825b4a) to head (4d17147).
⚠️ Report is 48 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2717      +/-   ##
==========================================
- Coverage   74.41%   72.92%   -1.49%     
==========================================
  Files          70       72       +2     
  Lines       39211    37285    -1926     
==========================================
- Hits        29177    27190    -1987     
- Misses      10034    10095      +61     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 58 untouched benchmarks
⏩ 6 skipped benchmarks¹

---

_{Comparing badumbatish:no_cxx_module (4d17147) with main (46e96ab)}

Footnotes

1. 6 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. ↩

[badumbatish]

please add a test

apologies for the late actions, please see the latest PR