[clang] [clang] Add `intrin0.h` header to mimic `intrin0.h` used by MSVC STL for clang-cl (PR #75711)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

Thank you for the timing information! The yvals_core.h hack is actually a really clever way to handle this. I'm a bit worried that we don't have any test coverage for this file to begin with, so it's a bit hard to validate that the changes are correct, but we're early enough in a release cycle that I think we'll get feedback with plenty of time to address unintentional fallout.

The changes should come with a release note in `clang/docs/ReleaseNotes.rst` so that users know about the compile time performance improvements. Otherwise, this LGTM! I can land the changes on your behalf once there's a release note.

https://github.com/llvm/llvm-project/pull/75711