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

[via cfe-commits]

YexuanXiao wrote:

> We don’t have to do anything in practice, [Xwin](https://github.com/Jake-Shadle/xwin) is already such a « script » and it already creates symlinks for all the casing issues and supports the /winsysroot layout if asked.

> What is the advantage of using clang (not CL)? We’ve been telling users for years to not do that? —sysroot could just use the /winsysroot layout.

First, using xwin still requires manually specifying confusing multiple include and lib paths. Second, to support libc++, the STL headers must be separated into is' own directory to prevent header conflicts. Therefore, this must be resolved jointly by Clang driver and the sysroot script.


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