[clang] Reapply "[Win/X86] Make _m_prefetch[w] builtins to avoid winnt.h conflicts (#115099)" (PR #138360)

[Reid Kleckner via cfe-commits]

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@@ -138,6 +138,12 @@ let Attributes = [Const, NoThrow, RequiredVectorWidth<256>], Features = "avx" in
   }
 }
 
+// PRFCHW
+let Features = "prfchw", Header = "intrin.h", Attributes = [NoThrow, Const] in {
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rnk wrote:

Looks like I actually had a pending change to go back to the mmx target feature, so my past self shared your concern. :) 

https://github.com/llvm/llvm-project/pull/138360