[llvm] Update cloning of the DirectXShaderCompiler repo to not include DXC tests (PR #122178)

[Brian Favela via llvm-commits]

bfavela wrote:

> > It does look like @llvm-beanz explicitly included the `-DLLVM_INCLUDE_TESTS` flag when he first implemented this, so it would be best to get an explicit sign off from him.
> 
> Yea, I'm not sure why I wrote it that way... I did give @bfavela the initial patch for this. Once Brian is comfortable I'm happy to merge this.

The issue I mentioned was just with building using msbuild (yes, yes). I think this can get safely merged.

https://github.com/llvm/llvm-project/pull/122178