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

[Phoebe Wang via cfe-commits]

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phoebewang wrote:

When AVX512BF16 is enabled, it's lowered to `vcvtneps2bf16`, https://godbolt.org/z/d5snYdGn4. Otherwise, it's scalarized with libcall. Since we change for intrinsic, AVX512BF16 is guaranteed. I don't see any risk here.

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