[clang] [X86] Enable constexpr on LZCNT & BMI intrinsics (PR #94161)

[Simon Pilgrim via cfe-commits]

RKSimon wrote:

@aniplcc if the x86 builtins handling is causing a problem - could we split this so the generic c code intrinsics (andn, blsr etc.) are done first?

https://github.com/llvm/llvm-project/pull/94161