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

[Aaron Ballman via cfe-commits]

================
@@ -1162,6 +1162,11 @@ class Sema final {
   /// CurContext - This is the current declaration context of parsing.
   DeclContext *CurContext;
 
+  SemaOpenACC &OpenACC() {
+    assert(OpenACCPtr);
----------------
AaronBallman wrote:

I think the assert is reasonable for now; someday we may want to investigate adding nullability attributes in appropriate places and removing some asserts like these where sensible.

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