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/clangcodemodelplugin.cpp

@@ -32,6 +32,7 @@
 #ifdef WITH_TESTS
 #  include "test/clangbatchfileprocessor.h"
 #  include "test/clangcodecompletion_test.h"
+#  include "test/clangdtests.h"
 #endif
 
 #include <coreplugin/actionmanager/actioncontainer.h>
@@ -204,6 +205,7 @@ QVector<QObject *> ClangCodeModelPlugin::createTestObjects() const
 {
     return {
         new Tests::ClangCodeCompletionTest,
+        new Tests::ClangdTests,
     };
 }
 #endif