[clang] [AMDGPU][Clang] Handle instantiation-dependent fence arguments (PR #214294)

[Shilei Tian via cfe-commits]

shiltian wrote:

> Do we even still need `__builtin_amdgcn_fence`? I was hoping `__scoped_atomic_thread_fence` would replace it, but that doesn't preclude us from fixing it.

Yeah, we can probably remove that, but the code refactoring here also makes the function much cleaner.

https://github.com/llvm/llvm-project/pull/214294