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

[Paweł Bylica via cfe-commits]

chfast wrote:

> But in future it would be useful to mark whole gap there as skipped instead. If there is interest I would do it in another PR.

This would be very useful not to treat compile-time unreachable code as uncovered. Can the skipped code be marked separately in the reports?

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