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

[Alex Voicu via cfe-commits]

https://github.com/AlexVlx commented:

> 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).

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