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

[via cfe-commits]

trcrsired wrote:

> This already works, as far as I know - through the `/winsysroot` and `-Xmicrosoft-windows-sys-root` options.
> 

/winsysroot was defined poorly to begin with although it is another story.

> > That would probably require more work, such as supporting case-insensitive includes, but seems like it would be much user friendlier.
> 
> Supporting case insensitive includes would indeed be nice.

I strongly oppose case insensitive includes. Linux kernel headers DO use the same insensitive include file names with case sensitive headers. It breaks code. You might say it can be just an option, but an option provide fragmentations and it will create more problems for testing, considering C++ headers situation are already bad enough.

include/linux/netfilter
<img width="867" height="464" alt="image" src="https://github.com/user-attachments/assets/b195d221-489c-4669-bed2-663d686f8052" />
xt_connmark.h vs xt_CONNMARK.h 

xt_tcpmss.h vs  xt_TCPMSS.h


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