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

[Saleem Abdulrasool via cfe-commits]

compnerd wrote:

>From what I can see from a cursory glance, there is some additional piece of content in this change: it is introducing a new `-std=stl` for using the MS STL as a standard C++ library, a new `vcruntime` parameter for `-rtlib=` and `-libunwind=`.

I think that adding support for STL is reasonable - STL and libc++ both are valid options on Windows for the C++ runtime. There are two separate ways to consider STL usage: as part of the MSVC installation or as a standalone build.

It would simplify things if we were to separate the concerns here. Lets split out the addition of the new flags for the unwind library, compiler runtime library, and C++ runtime. We can handle those additions separately. Adding them specifically with support for the MSVC layout and the MSVC flags gets us to a point where there is support for the functionality. From there, it is a question of how to get the argument handling mapped over properly.

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