[clang] [clang] Support --sysroot= for ${arch}-windows-msvc targets (PR #96417)

[via cfe-commits]

trcrsired wrote:

> Backing up a bit, where is the sysroot directory that this `--sysroot` flag for windows-msvc points to supposed to come from?
> 
> I thought this was for pointing clang to an MSVC installation, similar to clang-cl's /winsysroot, but it looks like the code is doing something else.

https://github.com/trcrsired/windows-msvc-sysroot/tree/main

I extract all files from msvc installations and windows SDK and put them in the format of what a normal sysroot would do on other platforms. Basically, unify the behavior with another platform. 

https://github.com/llvm/llvm-project/pull/96417