[clang] [OpenACC][CIR] Implement 'atomic capture' lowering (PR #168422)

[Henrich Lauko via cfe-commits]

================
@@ -324,30 +324,207 @@ OpenACCAtomicConstruct *OpenACCAtomicConstruct::Create(
   return Inst;
 }
 
-static std::pair<const Expr *, const Expr *> getBinaryOpArgs(const Expr *Op) {
+static std::optional<std::pair<const Expr *, const Expr *>>
+getBinaryAssignOpArgs(const Expr *Op, bool &isCompoundAssign) {
   if (const auto *BO = dyn_cast<BinaryOperator>(Op)) {
-    assert(BO->isAssignmentOp());
-    return {BO->getLHS(), BO->getRHS()};
+    if (!BO->isAssignmentOp())
+      return std::nullopt;
+    isCompoundAssign = BO->isCompoundAssignmentOp();
+    return std::pair<const Expr *, const Expr *>({BO->getLHS(), BO->getRHS()});
----------------
xlauko wrote:

```suggestion
    return std::pair<const Expr *, const Expr *>(BO->getLHS(), BO->getRHS());
```

https://github.com/llvm/llvm-project/pull/168422