[analyzer] Add a prunable note for skipping vbase inits in subclasses.

When initialization of virtual base classes is skipped, we now tell the user
about it, because this aspect of C++ isn't very well-known.

The implementation is based on the new "note tags" feature (r358781).
In order to make use of it, allow note tags to produce prunable notes,
and move the note tag factory to CoreEngine.

Differential Revision: https://reviews.llvm.org/D61817

llvm-svn: 361682