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

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trcrsired wrote:

<img width="569" height="724" alt="image" src="https://github.com/user-attachments/assets/bad8f63e-ea73-4727-a2a7-760d3a3bb59a" />
See case insensitive "same" file names with different cases?

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