[CodeComplete] Collect visited contexts when scope specifier is invalid.

Summary:
This will allow completion consumers to guess the specified scope by
putting together scopes in the context with the specified scope (e.g. when the
specified namespace is not imported yet).

Reviewers: ilya-biryukov

Subscribers: jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58446

llvm-svn: 354570