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

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trcrsired wrote:

> > > I've always kind of held out this idea that clang[++] are perfectly reasonable frontends to use when targeting an MSVC-environment with an official Microsoft Windows SDK
> > 
> > 
> > Me too, but isn't this PR a different idea? This is about using clang with MSVC/Win SDK turned into a Unix-style sysroot, which seems like a much narrower use case.
> > I think it would be better if the driver could work with the MSVC/SDK as-is, rather than forcing the user to change the SDK into a form that the driver likes (even if it's a "standard" form in the sense that other sysroots use it). That would probably require more work, such as supporting case-insensitive includes, but seems like it would be much user friendlier.
> > But I won't block this if there's otherwise strong support for it.
> 
> cross compiling is not a narrow use bro.
> 
> And this PR will allow using libc++ with microsoft sdk too and unfortunately microsoft puts C headers and MSVC STL headers together. Without a UNIX style sysroot that seperates msvc stl headers into a seperate include/c++/msvcstl there is no way to get it work.

in fact i use clang on my android phone to cross compile windows on arm .exe binaries and run eith wine every day. It is literally native compilation for me at this point.
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I think the reason you said it is narrow probably because you come from the background of big tech such as google which relies on libc++ on windows that is fine. But for small devs like me who need toolchains to run on android cross compilung is a huge big deal

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