[clang] [X86][Clang] Allow constexpr evaluation of F16C CVTPS2PH intrinsics (PR #162295)

[Phoebe Wang via cfe-commits]

phoebewang wrote:

> @phoebewang Are you OK with this? An alternative is we just allow exact rounding (which is what we'll end up doing for the SSE2 style conversions like CVTPD2DQ).

Can we check for strictfp mode? I think we can assume `NearestTiesToEven` in non strictfp mode.

https://github.com/llvm/llvm-project/pull/162295