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

[via cfe-commits]

trcrsired wrote:

Even Linus Torvalds agrees with that.
https://www.phoronix.com/news/Linus-Torvalds-Anti-Case-Fold
https://lore.kernel.org/lkml/CAHk-=wjajMJyoTv2KZdpVRoPn0LFZ94Loci37WLVXmMxDbLOjg@mail.gmail.com/

Please no more case insensitive mess thanks.

```
Case-insensitive names are horribly wrong, and you shouldn't have done
them at all. The problem wasn't the lack of testing, the problem was
implementing it in the first place.

The problem is then compounded by "trying to do it right", and in the
process doing it horrible wrong indeed, because "right" doesn't exist,
but trying to will make random bytes have very magical meaning.
Dammit. Case sensitivity is a BUG. The fact that filesystem people
*still* think it's a feature, I cannot understand. It's like they
revere the old FAT filesystem _so_ much that they have to recreate it
- badly.
```

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