[clang] clangCodeGen: Introduce `mcdc::State` with `MCDCState.h` (PR #81497)

[NAKAMURA Takumi via cfe-commits]

chapuni wrote:

@evodius96 Thanks. I can wait for other comments for a few days.

Considerations:
* `MCDCState.h` is enough small to be embeded into `CodeGenPGO.h`. I guess it nasty a bit if I did, though.
* `MCDCState.h` involves `CoverageMapping.h`, that is not small. It shall be replaced with smaller one with #81497.

https://github.com/llvm/llvm-project/pull/81497