[clang] Headers: prefer `corecrt_malloc.h` to `malloc.h` (PR #131668)

[Martin Storsjö via cfe-commits]

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

Looks ok to me.

I don't know about whether this really works as expected with `corecrt_malloc.h` or not, but I would expect you to have tested it.

We could look into adding `corecrt_malloc.h` in mingw-w64 as well, and then we could simplify this after a couple of years maybe.

https://github.com/llvm/llvm-project/pull/131668