TextEditor: Introduce shortcut for forcing a function hint proposal

... and support it in the ClangCodeModel.
This allows users to get function signature(s) displayed regardless of
where exactly the cursor is on the function call.

Fixes: QTCREATORBUG-19394
Change-Id: I033e8774db93680bfc3ee52610b817e0ef8ccc76
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cpptools/cppbuiltinmodelmanagersupport.cpp

@@ -74,6 +74,12 @@ CppCompletionAssistProvider *BuiltinModelManagerSupport::completionAssistProvide
     return m_completionAssistProvider.data();
 }
 
+
+CppCompletionAssistProvider *BuiltinModelManagerSupport::functionHintAssistProvider()
+{
+    return nullptr;
+}
+
 TextEditor::BaseHoverHandler *BuiltinModelManagerSupport::createHoverHandler()
 {
     return new CppHoverHandler;