[llvm] [SPIR-V] Fix LinkageAttribute emission for Vulkan (PR #143902)

[Steven Perron via llvm-commits]

s-perron wrote:

> > You are right; it does not. We might need to think of a different solution for variables.
> 
> Just to be sure, the case you want to cover is when generating an incomplete Vulkan SPIR-V module, which will then be linked to another before being used right? (Since this decoration/capability is simply never allowed for Vulkan)

Yes, that is how it is currently used in DXC. We do not want to take away this DXC feature. Here is one example of someone using it: https://github.com/microsoft/DirectXShaderCompiler/issues/7161. The Universal target env is just to work around the validator. That would not be needed for Clang since we do not run the validator as part of the compiler.

https://github.com/llvm/llvm-project/pull/143902