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

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

> > I don't think we should condone anything that assumes fetching WinSDK/MSVC derived files from a third party provider or github repo or anything like that.
> 
> I agree, but can we condone, or even redistribute, a script that post-processes the files this way, so anyone can replicate this setup for their personal use?

I don't see any legal issue wrt redistributing such a script, and it does indeed sound like a reasonable thing to have. So I don't mind us condoning such a tool.

> Looking at the linked github repo (which as you say may be violating a license)

Indeed, this aspect is one of my main concerns here.

> > Perhaps one thing we could do is maybe relax that assumption, and see if we can shift some of the burden from users to the driver by having some form of case-insensitive lookup when we are targeting `x86_64-pc-windows-msvc`?
> 
> I recall @compnerd asked for this at some point in the past, but perhaps he's just adopted the `ciopfs` mount solution. Anyway, [my search didn't find an existing issue](https://github.com/llvm/llvm-project/issues?q=is%3Aissue%20state%3Aopen%20case%20insensitive), so I'll file a new one for that. ... Filed as issue #161400 .

Thanks; if we'd manage to solve that (plus something similar for the linker...) that would make things a fair bit simpler already.

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