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

[via cfe-commits]

YexuanXiao wrote:

> In fact, `clang-cl` is exactly equal to `clang --driver-mode=cl`.

For downstream users, such as Visual Studio or CMake, they are different things.

> 1. libc++ when targeting `windows-msvc`. This is solved by directly using the flag, but then removing the C standard library—that is, `-nostdinc`, sending the driver into CL mode, and using the cl-style flag to set up the Windows SDK and UCRT directories;

UCRT and STL are provided in the same directory, how do you distinguish between them?



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