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

[Reid Kleckner via cfe-commits]

rnk wrote:

> The build is still broken (e.g. https://lab.llvm.org/buildbot/#/builders/63/builds/3861). I'll back it out.

Thank you for doing that. I missed all notifications because they got lost in the storm of failures due to the mismatch between my local build and what was on github.

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