Introduce KDevClangPrivate shared lib
Merge all static libs into one shared library. This fixes ODR violations
we introduced when linking static libs both into the plugin + unit
tests.
Summary: Also link plugin to it, fix exports and make tests link to the shared lib.
Test Plan: test_problems is working now on Windows
Reviewers: kfunk, KDevelop
Subscribers: kfunk, mwolff, kdevelop-devel
Differential Revision: https://phabricator.kde.org/D772