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

[Simon Pilgrim via cfe-commits]

RKSimon 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.

@ericxu233 ping?

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