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

[via cfe-commits]

trcrsired wrote:

> It's perfectly possible to use `clang/clang++` to compile with a Microsoft SDK already, on Windows and cross compiling. The normal method of accessing the SDK (through the `INCLUDE` and `LIB` env variables for specifying locations) works just like it does for `clang-cl` - the flavour of driver doesn't make any difference there.

Environmental variables to change how the functionalities are working are very bad idea. There are so many things that can affect it in a very surprise way. It will also force scripts to guard them (just like a lot of scripts have to use LC_ALL="C.UTF-8" all the time). it is really bad.

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