ClangCodeModel: Provide tooltips via clangd

Note that we temporarily lose the ability to hover over an include and
get the full path of the header file. This is a valuable feature that we
need to restore, preferably by fixing clangd itself.
Fixing the remaining few test failures would likely require more
complicated code as well as additional LSP round-trips, and as of now
I'm not convinced it is worth the effort.

Change-Id: I08c72c4bd1268bbd67baeb57bbfd29d9b11303a5
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangdclient.h

@@ -70,12 +70,17 @@ public:
     void findLocalUsages(TextEditor::TextDocument *document, const QTextCursor &cursor,
                          CppTools::RefactoringEngineInterface::RenameCallback &&callback);
 
+    void gatherHelpItemForTooltip(
+            const LanguageServerProtocol::HoverRequest::Response &hoverResponse,
+            const LanguageServerProtocol::DocumentUri &uri);
+
     void enableTesting();
 
 signals:
     void indexingFinished();
     void foundReferences(const QList<Core::SearchResultItem> &items);
     void findUsagesDone();
+    void helpItemGathered(const Core::HelpItem &helpItem);
 
 private:
     void handleDiagnostics(const LanguageServerProtocol::PublishDiagnosticsParams &params) override;