[clang] [Headers][X86] Convert bf16 to f32 conversions to constexpr implementations (PR #169841)

[Simon Pilgrim via cfe-commits]

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RKSimon wrote:

I'm hesitant to use `__builtin_convertvector` as although it looks like we're consistent with fp32 -> bf16 using round rmNearestTiesToEven in dag, I don't know the code well enough to be sure that we do this everywhere - do you think its ok?

https://github.com/llvm/llvm-project/pull/169841