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

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trcrsired wrote:

> FYI, MinGW can’t keep up with Windows SDK. The reason people still rely on MinGW today is largely because Windows SDK’s directory structure is very complicated for cross-compilation. This PR makes it possible for users who want to move away from MinGW to switch to Windows SDK - there’s huge potential demand in this PR.

thank you!

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