use llvm-cov gcov in coverage

[oliverlee]

Use of GCOV format, instead of LLVM profdata, results in a much lower
false-positive rate for lines with only comments.

Change-Id: I947e6a91ae6a35f9dc649660f16f066f106b7b11

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.81%. Comparing base (ae110a9) to head (4843eca).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #92      +/-   ##
==========================================
- Coverage   89.85%   89.81%   -0.05%     
==========================================
  Files          27       26       -1     
  Lines         345      324      -21     
  Branches       12       35      +23     
==========================================
- Hits          310      291      -19     
+ Misses         27        4      -23     
- Partials        8       29      +21     

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