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

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trcrsired wrote:

> > no. I do not want that complicated file structure. I just want normal GNU style's sysroot.
> > x86_64-windows-msvc sysroot should have exact the same file struct as x86_64-windows-gnu.
> 
> The -msvc triples are designed to work with libraries from MSVC and the Windows SDK. We can't change how those are organized.

There is no "--sysroot" for msvc and windows SDK. Also there is no support for libc++ in that directory either. My definition is correct to make clang --sysroot work the same way as other platforms.

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