Reapply "[NFC] Change the return type of getTraversalScope to ArrayRef"

vitalybuka identified a fix here that fixes the issue, and lets us make
fewer copies!  This applies his patch plus reapplys the original.

This reverts commit c4c29b9.

Author: erichkeane

clang-tools-extra/clangd/refactor/tweaks/AnnotateHighlightings.cpp

@@ -51,7 +51,8 @@ Expected<Tweak::Effect> AnnotateHighlightings::apply(const Selection &Inputs) {
         *Inputs.AST, /*IncludeInactiveRegionTokens=*/true);
   } else {
     // Store the existing scopes.
-    const auto &BackupScopes = Inputs.AST->getASTContext().getTraversalScope();
+    const std::vector<Decl *> BackupScopes =
+        Inputs.AST->getASTContext().getTraversalScope();
     // Narrow the traversal scope to the selected node.
     Inputs.AST->getASTContext().setTraversalScope(
         {const_cast<Decl *>(CommonDecl)});

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