Add unit test for DUChainUtils::getInheriters.

Summary:
This test should be placed in kdevplatform, but it was simpler to make
it here and use the clang language plugin for duchain creation.

Reviewers: KDevelop, mwolff, kfunk

Reviewed By: KDevelop, mwolff, kfunk

Subscribers: mwolff, kdevelop-devel

Projects: KDevelop

Differential Revision: https://phabricator.kde.org/D584