Add support for llvm 21

[bkryza]

Fixes #419

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.12%. Comparing base (378ea0c) to head (7fcda6a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #420   +/-   ##
=======================================
  Coverage   90.12%   90.12%           
=======================================
  Files         153      153           
  Lines       19240    19240           
=======================================
  Hits        17341    17341           
  Misses       1899     1899           

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