Summary:
This would allow code completion clients to know which context is visited during Sema code completion.
Also some changes:
* add `EnteredContext` callback in VisibleDeclConsumer.
* add a simple unittest for sema code completion (only for visited contexts at the moment).
Reviewers: ilya-biryukov
Reviewed By: ilya-biryukov
Subscribers: mgorny, bkramer, cfe-commits
Differential Revision: https://reviews.llvm.org/D42071
llvm-svn: 322661
