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

[via cfe-commits]

trcrsired wrote:

> > 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.
> 
> Isn't it better to point Clang at MSVC and the Windows SDK as they're normally installed?
> 
> This patch seems specific to your custom setup, not something that would be useful for others.

Windows SDK is not portable since it does not follow the unix naming convention.

No. I just create the same file structure as clang usually does. Plus clang itself does not support libc++ because there is no place to put LLVM libc++

```
/include: headers
/include/c++/stl: MSVC STL headers
/include/c++/v1: LLVM libc++
/lib/arch-unknown-windows-gnu: target specific libraries
/lib: generic libraries
```

This is the same rule as other triplets. I don't see how is that not useful because every other triplets for example x86_64-windows-gnu, x86_64-linux-gnu, wasm(32/64)-wasip1, aarch64-linux-android{version} are doing exactly the same

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