[clang] [clang][CodeGen][SPIR-V] Fix incorrect SYCL usage, implement missing interface (PR #109415)

[Arvind Sudarsanam via cfe-commits]

asudarsa wrote:

> > Are there any tests available to check this behavior?
> 
> The reworked tests do verify / rely on this behaviour, but I can add an individual test for both vanilla and AMDGCN flavoured SPIR-V, if that is preferred (might be better anyway).

Thanks @AlexVlx 
I will approve this change. You can add the tests as part of this PR or a follow-up PR. Either way is fine.
Sincerely

https://github.com/llvm/llvm-project/pull/109415