Add Clang 21 CI build

[offa]

CI build for the latest Clang release.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 34.28%. Comparing base (fb67abe) to head (a88ee39).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #212      +/-   ##
==========================================
+ Coverage   33.99%   34.28%   +0.28%     
==========================================
  Files          52       53       +1     
  Lines        2271     2281      +10     
  Branches      336      340       +4     
==========================================
+ Hits          772      782      +10     
  Misses       1419     1419              
  Partials       80       80              

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[mattgodbolt]

thanks @offa