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

[via cfe-commits]

trcrsired wrote:

> > Let's do not make things worse by creating more fragmentations
> 
> But this PR does precisely that, by expecting a non-standard directory layout for `--sysroot`. By non-standard I mean modified from what the vendor provides. It might be a 'de-facto standard for UNIX' but it is _not_ the standard for Windows.
> 
> If the goal is to get `--sysroot` to expect `{arch}-pc-windows-msvc` then I believe the correct solution is to alias it to `-Xmicrosoft-windows-sys-root` when the argument value is `{arch}-pc-windows-msvc`, and ensure case-insensitivity is correctly handled in the driver. As it is, this PR is therefore incomplete.

1. POSIX is the OS standard.
2. MinGW does this. CYGWIN does this. MSYS2 does this. Every windows platform does it.
3. Case insensitivity is a filesystem thing. In fact ntfs and refs allows case sensitive too. You are really just breaking code by doing so.

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