[CIR][AMDGPU][NFC] Add CIRGenBuiltinAMDGPU file to support AMDGPU builtins codegen (#179237)
Upstreaming ClangIR PR : https://github.com/llvm/clangir/pull/2030
This PR adds CIRGenBuiltinAMDGPU.cpp file for AMDGPU specific builtin
codegen support.
Lists out all the builtins that are currently supported for codegen in
`clang/lib/CodeGen/TargetBuiltins/AMDGPU.cpp`.
All builtins codegen are currently "NYI".