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/clangcodemodel/clangcompletionassistprovider.cpp

@@ -43,8 +43,9 @@
 namespace ClangCodeModel {
 namespace Internal {
 
-ClangCompletionAssistProvider::ClangCompletionAssistProvider(BackendCommunicator &communicator)
-    : m_communicator(communicator)
+ClangCompletionAssistProvider::ClangCompletionAssistProvider(BackendCommunicator &communicator,
+                                                             CompletionType type)
+    : m_communicator(communicator), m_type(type)
 {
 }
 
@@ -68,6 +69,7 @@ TextEditor::AssistInterface *ClangCompletionAssistProvider::createAssistInterfac
     const CppTools::ProjectPart::Ptr projectPart = projectPartForFileBasedOnProcessor(filePath);
     if (projectPart) {
         return new ClangCompletionAssistInterface(m_communicator,
+                                                  m_type,
                                                   textEditorWidget,
                                                   position,
                                                   filePath,