C++: show function declarations in the locator.

When searching for functions/methods in the locator, also show function
and method declarations.

Task-number: QTCREATORBUG-8862
Change-Id: Ie86bfe82403d79a903d769860a472e2a20a2b9d0
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>