[clang] [CIR][AMDGPU][NFC] Add CIRGenBuiltinAMDGPU file to support AMDGPU builtins codegen (PR #179237)

[Andy Kaylor via cfe-commits]

================
@@ -1902,6 +1885,7 @@ emitTargetArchBuiltinExpr(CIRGenFunction *cgf, unsigned builtinID,
   case llvm::Triple::ppc64le:
   case llvm::Triple::r600:
----------------
andykaylor wrote:

```suggestion
  case llvm::Triple::r600:
    return std::nullopt;
```
This shouldn't fall through. Maybe also duplicate the comment from line 1897.

https://github.com/llvm/llvm-project/pull/179237