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

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trcrsired wrote:

> @trcrsired, I don't think it was said that 'cross-compilation is a narrow scenario', instead it was 'making the Windows SDK and MSVC STL into a Unix-like sysroot' is a narrow scenario, which is broadly true.

It is not true. Or LLVM itself won't package this way on windows. Blocking PR like this proves my point you said "cross compilation is a narrow scenario" since literally this is the only sound solution i have tested.

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