[clang] [clang][coverage] fixing "if constexpr" and "if consteval" coverage report (PR #77214)

[Hana Dusíková via cfe-commits]

hanickadot wrote:

<img width="906" alt="compare" src="https://github.com/llvm/llvm-project/assets/6557263/47ff77c0-9101-44cf-b2d5-ffea514bfc0c">

(notice wrong coverage "if constexpr" for positive one, and completely missing for negative one, also notice "if consteval" marking always the same branch as uncovered)



https://github.com/llvm/llvm-project/pull/77214