Workaround: find declarations for constructor argument hints

Summary:
For non-namespaced constructors we can find the declarations by
manually adding the identifier a second time before the lookup,
as that gives us the proper QID. This allows us to find the
declaration for many situations, most notably for most Qt classes.

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D9745