[Completion] Code complete the members for a dependent type after a '::'

This commit is a follow up to r302797 which added support for dependent
completions after the '.' and '->' operators. This commit adds support for
dependent completions after the '::' operator.

Differential Revision: https://reviews.llvm.org/D34173


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@305511 91177308-0d34-0410-b5e6-96231b3b80d8