[clang] [llvm] [clang][CoverageMapping] Refactor when setting MC/DC True/False (PR #78202)

[NAKAMURA Takumi via cfe-commits]

https://github.com/chapuni approved this pull request.

Thanks!

I recommend you to commit `llvm` part as the separated one in advance, if you could commit w/o pull requests.

https://github.com/llvm/llvm-project/pull/78202