[clang-rename] Respect the traversal scope when traversing the entire AST.

[hokein]

Summary

This should be NFC to clang-rename, by default the traversal scope is
TUDecl. Traversing the TUDecl in clangd is a performance cliff, we should
avoid it.

[ilya-biryukov]

LGTM

clang/lib/Tooling/Refactoring/Rename/USRFindingAction.cpp

 // to virtual method.
 class AdditionalUSRFinder : public RecursiveASTVisitor<AdditionalUSRFinder> {
 public:
   AdditionalUSRFinder(const Decl *FoundDecl, ASTContext &Context)
       : FoundDecl(FoundDecl), Context(Context) {}
 
   std::vector<std::string> Find() {
     // Fill OverriddenMethods and PartialSpecs storages.
-    TraverseDecl(Context.getTranslationUnitDecl());
+    TraverseAST(Context);
     if (const auto *MethodDecl = dyn_cast<CXXMethodDecl>(FoundDecl)) {
       addUSRsOfOverridenFunctions(MethodDecl);
       for (const auto &OverriddenMethod : OverriddenMethods) {
         if (checkIfOverriddenFunctionAscends(OverriddenMethod))
           USRSet.insert(getUSRForDecl(OverriddenMethod));
       }
       addUSRsOfInstantiatedMethods(MethodDecl);
     } else if (const auto *RecordDecl = dyn_cast<CXXRecordDecl>(FoundDecl)) {