[clang] [llvm] [MC/DC] Refactor: Make `MCDCParams` as `std::variant` (PR #81227)

[NAKAMURA Takumi via cfe-commits]

https://github.com/chapuni commented:

@ornata I want to isolate Decision stuff and Branch stuff in `MCDCParams`.
Also I'd like to encapsulate params into each record. Once parameters are set, they are expected to hold "valid" values. Zeroing really confused me, (possibly us).

I've found `std::variant` works good for this area. I expect it will mitigate confusions little a bit.

https://github.com/llvm/llvm-project/pull/81227