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/clangdclient.h

@@ -27,7 +27,11 @@
 
 #include <languageclient/client.h>
 
+#include <QVersionNumber>
+
+namespace Core { class SearchResultItem; }
 namespace ProjectExplorer { class Project; }
+namespace TextEditor { class TextDocument; }
 
 namespace ClangCodeModel {
 namespace Internal {
@@ -37,11 +41,26 @@ class ClangdClient : public LanguageClient::Client
     Q_OBJECT
 public:
     ClangdClient(ProjectExplorer::Project *project, const Utils::FilePath &jsonDbDir);
+    ~ClangdClient() override;
 
-    bool isFullyIndexed() const { return m_isFullyIndexed; }
+    bool isFullyIndexed() const;
+    QVersionNumber versionNumber() const;
+
+    void openExtraFile(const Utils::FilePath &filePath, const QString &content = {});
+    void closeExtraFile(const Utils::FilePath &filePath);
+
+    void findUsages(TextEditor::TextDocument *document, const QTextCursor &cursor);
+
+    void enableTesting();
+
+signals:
+    void indexingFinished();
+    void foundReferences(const QList<Core::SearchResultItem> &items);
+    void findUsagesDone();
 
 private:
-    bool m_isFullyIndexed = false;
+    class Private;
+    Private * const d;
 };
 
 } // namespace Internal