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

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Andarwinux wrote:

In fact, a major factor in the success of LLVM was precisely the abandonment of the GPL and the adoption of a more liberal license. Today the main contributors to LLVM are from corporations. So I think for this PR to be recognized, it has to get major corporations like Google interested in it.

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