[clang] [clang] Factor out OpenACC part of `Sema` (PR #84184)

[Sam McCall via cfe-commits]

================
@@ -63,17 +67,17 @@ void Sema::ActOnOpenACCConstruct(OpenACCDirectiveKind K,
     // here as these constructs do not take any arguments.
     break;
   default:
-    Diag(StartLoc, diag::warn_acc_construct_unimplemented) << K;
+    SemaRef.Diag(StartLoc, diag::warn_acc_construct_unimplemented) << K;
----------------
sam-mccall wrote:

Agreed. Is there a better way than this?

```
class SemaComponent {
  Sema &S;
protected:
  SemaComponent(Sema &S) : S(S) {}
  Sema& sema() { return S; }
  SemaDiagnosticBuilder Diag(...) { return sema().Diag(...); }
  // ...
};

class SemaOpenACC : public SemaComponent {
public:
  using SemaComponent::SemaComponent;
  ...
}
```

Accessing `sema()` would be a marker of crossing component boundaries, which is worthy of some scrutiny.

This requires some agreement on what common functionality is. On the other hand it forces some discussion/review of this definition which seems useful.

It requires SemaOpenACC.h to depend on Sema.h (which personally I think is reasonable, based on what clients will do).

It also requires a bunch of boilerplate, which is sad.

https://github.com/llvm/llvm-project/pull/84184