[clang] [Win/X86] Make _m_prefetch[w] builtins to avoid winnt.h conflicts (PR #115099)

[James Y Knight via cfe-commits]

jyknight wrote:

(...and maybe we could also get rid of the similar hacks we did for _mm_mfence/etc before? The commit message for 727ab8a80346eeec39e13293f5dc01cc82bb1d95 didn't say anything about the rationale, but if it's the same as this, then perhaps so?)

https://github.com/llvm/llvm-project/pull/115099