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

[Nathan Gauër via llvm-commits]

Keenuts 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)

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