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

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

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

Thanks, I was just about to report the same issue as well; this seems to conflict with winnt.h, at least in older versions of WinSDK (10.0.18362.0 in my case too). I don't have more data to go on, other than what's in that build log:

```
In file included from C:\b\slave\clang-x64-windows-msvc\llvm-project\llvm\lib\Support\COM.cpp:21:
In file included from C:\b\slave\clang-x64-windows-msvc\llvm-project\llvm\lib\Support\Windows/COM.inc:17:
In file included from C:\Program Files (x86)\Windows Kits\10\include\10.0.18362.0\um\objbase.h:15:
In file included from C:\Program Files (x86)\Windows Kits\10\include\10.0.18362.0\shared\rpc.h:20:
In file included from C:\Program Files (x86)\Windows Kits\10\include\10.0.18362.0\um\windows.h:171:
In file included from C:\Program Files (x86)\Windows Kits\10\include\10.0.18362.0\shared\windef.h:24:
In file included from C:\Program Files (x86)\Windows Kits\10\include\10.0.18362.0\shared\minwindef.h:182:
C:\Program Files (x86)\Windows Kits\10\include\10.0.18362.0\um\winnt.h(3318,1): error: conflicting types for '_mm_prefetch'
 3318 | _mm_prefetch (
      | ^
C:\Program Files (x86)\Windows Kits\10\include\10.0.18362.0\um\winnt.h(3318,1): note: '_mm_prefetch' is a builtin with type 'void (const void *, int) noexcept'
1 error generated.
```

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