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

[via cfe-commits]

YexuanXiao wrote:

@sharadhr @aganea @Andarwinux @zmodem I suggest we focus on the following two points to avoid getting bogged down in irrelevant discussions raised by the author:
1. Regardless of the libraries and header files, does this patch **make Clang's API more consistent**, making it easier for downstream users to use them in a unified way? (I verified that it works well with CMake.)  
2. Can the script used to generate this sysroot be **maintained as part of Clang**, allowing users to build it themselves to avoid licensing issues?

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