[clang] [Driver] Support libc++ in MSVC (PR #219260)

[Alexandre Ganea via cfe-commits]

aganea wrote:

Shouldn't we provide (as a follow-up to this PR, or preceding this PR) a `SubtractiveFileSystem` + cc1 flag that removes from the VFS view all STL files from the MS-STL folder? We could keep in that VFS view any non-STL libraries (`ppl.h`, `concurrent*.h`, `comdef.h`, etc.) and all the CRT / vcruntime files. I think the concerns raised in [this RFC thread](https://discourse.llvm.org/t/rfc-support-sysroot-for-arch-windows-msvc-targets/91650/15) are real. Whether MS-STL files or libc++ files are visible to the compiler should be a dynamic selection controlled by the flag proposed by this PR.

https://github.com/llvm/llvm-project/pull/219260