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

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trcrsired wrote:

@yxsamliu #207798
Is it msvcstl or msstl? 
LLVM thought it should be msstl instead. Because it is clearly Microsoft/STL project.

i will change to msvcstl if you think that is more apporiate, unfortunately it breaks code.
https://github.com/trcrsired/windows-msvc-sysroot/tree/main/include/c%2B%2B/msstl 

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