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

[via cfe-commits]

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.

case insensitivity (similar things such as EBCDIC, wchar_t) was a historical mistake to begin with.  Let's do not make things worse by creating more fragmentations.

Also the argument of packaing as UNIX style is too much ask would be true for basically any targets over there. You always need to build libc and install it to create a UNIX style sysroot. In fact even llvm itself uses UNIX style sysroot packaging on windows.

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