[NFC] Change the return type of getTraversalScope to ArrayRef

This is only used in 2 places, both just to search through the list, so
returning a copy was wasteful.  This patch changes it to return an
`ArrayRef` so that a copy isn't made.

Author: erichkeane

clang/include/clang/AST/ASTContext.h

@@ -723,7 +723,7 @@ class ASTContext : public RefCountedBase<ASTContext> {
   // (However they are still accessible via TranslationUnitDecl->decls())
   //
   // Changing the scope clears the parent cache, which is expensive to rebuild.
-  std::vector<Decl *> getTraversalScope() const { return TraversalScope; }
+  ArrayRef<Decl *> getTraversalScope() const { return TraversalScope; }
   void setTraversalScope(const std::vector<Decl *> &);
 
   /// Forwards to get node parents from the ParentMapContext. New callers should