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

[via cfe-commits]

YexuanXiao wrote:

This patch is useful. This is a simple way to get rid of the huge Visual Studio installation. I use the following command line to configure my project with cmake and use libc++ as the standard library on Windows:
```powershell
cmake -G Ninja -D "CMAKE_C_COMPILER=D:\Projects\clang\llvm\bin\clang.exe" -D "CMAKE_CXX_COMPILER=D:\Projects\clang\llvm\bin\clang++.exe" -D "CMAKE_C_FLAGS=--target=x86_64-windows-msvc --sysroot=D:\Projects\windows-msvc-sysroot -fuse-ld=lld" -D "CMAKE_CXX_FLAGS=--target=x86_64-windows-msvc --sysroot=D:\Projects\windows-msvc-sysroot -fuse-ld=lld -stdlib=libc++" ..
```
By changing -stdlib=libc++ to -stdlib=stl, I can use stl as the C++ standard library.

Due to cmake [incorrectly](https://gitlab.kitware.com/cmake/cmake/-/issues/20705) using msvc-style lld-link instead of lld when using gnu-sysroot, manual compilation is currently the only way to use gnu-sysroot:
```powershell
clang++ --target=x86_64-windows-gnu --sysroot=D:\Projects\clang\x86_64-windows-gnu -rtlib=compiler-rt --unwindlib=libunwind -fuse-ld=lld -stdlib=libc++ -lc++abi -lunwind -std=c++23 examples.cpp
```

Overall, I believe this patch is effective, although windows-msvc-sysroot is a private repository, it doesn't have any conceptual differences from other sysroots.  Perhaps the author should create a tutorial on how to build this system root.

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