[llvm] [SPIR-V] Fix ExecutionMode generation (PR #143888)

[Nathan Gauër via llvm-commits]

Keenuts wrote:

> > > Can you add a test for this?
> > 
> > 
> > This requires a fragment shader with an input. Found it while building `vk::location`. For now we have no input on FS which builds correctly. Once we have a test using `vk::location` with FS, this will be covered
> 
> The problem seems to describe the interface variables being added. In later versions of SPIR-V, this includes resources. Can't you generate a sample with a storage buffer or something?

Oh right, I can get an interface variable from a buffer. I'll add this then.

https://github.com/llvm/llvm-project/pull/143888