[Sema] Add visited contexts to CodeCompleteContext

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