[libcxx-commits] [clang-tools-extra] [lldb] [llvm] [openmp] [clang] [compiler-rt] [libcxx] [lld] [flang] [mlir] [clang] Add `intrin0.h` header to mimic `intrin0.h` used by MSVC STL for clang-cl (PR #75711)

[Max Winkler via libcxx-commits]

MaxEW707 wrote:

> #include <memory>
>
>clang-cl 16 frontend took ~190ms to parse those 3 headers. intrin.h took ~32ms to parse.
>
>clang-cl built with this PR frontend took ~1368ms to parse. intrin.h took ~969ms to parse. Most of that time is from <x86intrin.h> as expected. intrin0.h took ~2ms to parse.

Did this same test with the most recent change that included the `yvals_core.h` interposing header.
The parse time was around ~170ms since `intrin0.h` is still cheaper than the old `intrin.h`.

This was tested with the MSVC STL shipped with MSVC 1938.

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