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/texteditor/texteditor.h

@@ -491,6 +491,10 @@ public:
     void addHoverHandler(BaseHoverHandler *handler);
     void removeHoverHandler(BaseHoverHandler *handler);
 
+#ifdef WITH_TESTS
+    void processTooltipRequest(const QTextCursor &c);
+#endif
+
 signals:
     void assistFinished(); // Used in tests.
     void readOnlyChanged();