[llvm] [SPIR-V] Map SPIR-V friendly work-item function to built-in variables (PR #148567)

[Victor Lomuller via llvm-commits]

Naghasan wrote:

We probably need to align the names https://github.com/llvm/llvm-project/blob/main/clang/lib/Headers/__clang_spirv_builtins.h#L54

https://github.com/llvm/llvm-project/pull/148567