Document lack of clang support [skip ci]
Document lack of clang support [skip ci]

Meson doesn't currently provide a very helpful message when trying to generate a coverage report with clang, and in fact just silently fails for 2 of the 3 reports.  Ideally Meson would support coverage with llvm-cov, or provide a more helpful error message.  Until then, it seems it would be helpful to at least put a warning in the documentation
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