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

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `premerge-monolithic-linux` running on `premerge-linux-1` while building `clang` at step 6 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/153/builds/21790

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (build-unified-tree) failure: build (failure)
0.004 [3613/8/1] Copying clang's prfchwintrin.h...
0.004 [3613/7/2] Copying clang's xmmintrin.h...
0.008 [3613/6/3] Performing build step for 'bolt_rt'
ninja: no work to do.
0.012 [3612/6/4] No install step for 'bolt_rt'
0.014 [3611/6/5] Building BuiltinsX86.inc...
FAILED: tools/clang/include/clang/Basic/BuiltinsX86.inc /build/buildbot/premerge-monolithic-linux/build/tools/clang/include/clang/Basic/BuiltinsX86.inc 
cd /build/buildbot/premerge-monolithic-linux/build && /build/buildbot/premerge-monolithic-linux/build/bin/clang-tblgen -gen-clang-builtins -I /build/buildbot/premerge-monolithic-linux/llvm-project/clang/include/clang/Basic -I/build/buildbot/premerge-monolithic-linux/llvm-project/clang/include -I/build/buildbot/premerge-monolithic-linux/build/tools/clang/include -I/build/buildbot/premerge-monolithic-linux/build/include -I/build/buildbot/premerge-monolithic-linux/llvm-project/llvm/include /build/buildbot/premerge-monolithic-linux/llvm-project/clang/include/clang/Basic/BuiltinsX86.td --write-if-changed -o tools/clang/include/clang/Basic/BuiltinsX86.inc -d tools/clang/include/clang/Basic/BuiltinsX86.inc.d
/build/buildbot/premerge-monolithic-linux/llvm-project/clang/include/clang/Basic/BuiltinsX86.td:147:7: error: Unexpected header name
  def _m_prefetch : X86LibBuiltin<"void(void *)">;
      ^
0.014 [3611/5/6] Building CXX object tools/llvm-config/CMakeFiles/llvm-config.dir/llvm-config.cpp.o
0.017 [3611/4/7] Completed 'bolt_rt'
0.017 [3611/3/8] Generating VCSRevision.h
0.019 [3611/2/9] Generating VCSVersion.inc
0.028 [3611/1/10] Generating VCSVersion.inc
ninja: build stopped: subcommand failed.

```

</details>

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