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

[via cfe-commits]

zmodem wrote:

Backing up a bit, where is the sysroot directory that this `--sysroot` flag for windows-msvc points to supposed to come from?

I thought this was for pointing clang to an MSVC installation, similar to clang-cl's /winsysroot, but it looks like the code is doing something else.

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