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

[Simon Pilgrim via cfe-commits]

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RKSimon wrote:

The docs indicate these always round to nearest even - we could just handle the builtins in constexpr - similar to how we handle PS2PH?

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