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

[via cfe-commits]

LW-archlinux wrote:

Commit 75f0d40507ea3f7c99dd3250ff0fbe6dab341910 breaks build for a multilib 32-bit clang build.

See https://github.com/llvm/llvm-project/issues/81880

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