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

[Reid Kleckner via cfe-commits]

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

I think there are risks to shadowing `yvals_core.h`, but @CaseyCarter is at least aware of it [here](https://github.com/microsoft/STL/issues/3634#issuecomment-1904956652).

Once Clang ships its own intrin0.h header, the MSVC STL can adjust their yvals_core.h code to use it, and then we can delete this shadow header. This gives us the best outcome: Fast compiles today, target attribute support in MSVC-mode, and minimal tech debt in the long run. Sounds good to me, thanks for finding a solution!

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