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

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YexuanXiao wrote:

> Hi,
> 
> I also hope this PR can be merged, but considering the other comments, I don’t think this is the right direction.
> 
> I think what we might want is not "Support --sysroot= for ${arch}-windows-msvc targets," but a new target, maybe ${arch}-windows-msvcllvm, a.k.a. "MinCW"? This target is supposed to be fully ABI-compatible with MSVC, while it provides the GNU Clang user interface and directory layout. On top of fully ABI-compatible with MSVC, it is supposed to optionally supplement and replace the MSVC CRT/STL with high-quality LLVM runtimes.
> 
> I believe Chromium might be interested in this, which would greatly simplify their build system.
> 
> 

I think it is possible to generate this sysroot by providing a Python script that uses the existing WindowsSDK and MSVC toolchain on the local drive, so there is no need to send header files and libraries to users, avoiding licence issues. However, I currently don't have much interest in it because it feels somewhat self-deceptive. In fact, the patch works very well, it not only builds programs correctly but also work well with cmake and clangd.

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