Avoid duplicates in virtual overrides code completion proposals

Summary:
Currently methods which have multiple overrides in the inheritance
chain are also displayed as often. Filtering out these duplicates
based on the signature improves the situation as a first step.

Test Plan:
E.g. QWidget subclasses have lots of methods repeated, with this
patch no longer.
Extended unit test failed before, no longer fails with patch.

Reviewers: KDevelop, mwolff

Reviewed By: KDevelop, mwolff

Subscribers: mwolff, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D6417