ClangCodeModel: Support access type categorization

... with "Find Usages", as we do in the built-in code model.
Note 1: This is very slow, so it's for now only enabled if the
        search results come from a small number of files.
        Possible ways of speeding up the operation
        to be investigated.
Note 2: All test cases from the old code model also pass here,
        but checking with non-trivial real-world projects
        shows a lot of mis-categorizations.
        Well will fix them one by one.
Note 3: This functionality requires clangd >= 13.

Change-Id: Ib3500b52996dbbf9d7d9712d729179bcbd3262fc
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangrefactoringengine.cpp

@@ -97,11 +97,9 @@ void RefactoringEngine::findUsages(const CppTools::CursorInEditor &cursor,
                 ->findUsages(cursor, std::move(callback));
         return;
     }
-    // TODO: We want to keep our "access type info" feature.
-    //       Check whether we can support it using clang 12's textDocument/ast request
     if (!client->documentOpen(cursor.textDocument()))
         client->openDocument(cursor.textDocument()); // TODO: Just a workaround
-    client->symbolSupport().findUsages(cursor.textDocument(), cursor.cursor());
+    client->findUsages(cursor.textDocument(), cursor.cursor());
 }
 
 } // namespace Internal