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/clangcodemodel.qbs

@@ -8,6 +8,7 @@ QtcPlugin {
     Depends { name: "Core" }
     Depends { name: "CppTools" }
     Depends { name: "ProjectExplorer" }
+    Depends { name: "QtSupport"; condition: qtc.testsEnabled }
     Depends { name: "TextEditor" }
     Depends { name: "Utils" }
     Depends { name: "ClangSupport" }
@@ -114,6 +115,8 @@ QtcPlugin {
             "clangbatchfileprocessor.h",
             "clangcodecompletion_test.cpp",
             "clangcodecompletion_test.h",
+            "clangdtests.cpp",
+            "clangdtests.h",
             "data/clangtestdata.qrc",
         ]
     }