[clang] [OpenACC][NFC] Add OpenACC Clause AST Nodes/infrastructure (PR #87675)

[Alexey Bataev via cfe-commits]

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@@ -30,13 +31,23 @@ class OpenACCConstructStmt : public Stmt {
   /// the directive.
   SourceRange Range;
 
-  // TODO OPENACC: Clauses should probably be collected in this class.
+  /// The list of clauses.  This is stored here as an ArrayRef, as this is the
+  /// most convienient place to access the list, however the list itself should
+  /// be stored in leaf nodes, likely in trailing-storage.
+  MutableArrayRef<const OpenACCClause *> Clauses;
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alexey-bataev wrote:

Why need to keep as MutbleArrayRef, not just ArrayRef?

https://github.com/llvm/llvm-project/pull/87675