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

[Reid Kleckner via cfe-commits]

https://github.com/rnk commented:

I had to rebase this over the table-gen-ification of the builtins, but I think this is still relevant.

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