[change-namespace] add leading '::' to references in new namespace when name conflict is possible.

Summary:
For example, when we change 'na' to "nb::nc", we need to add leading '::' to
references "::nc::X" in the changed namespace.

Reviewers: bkramer

Reviewed By: bkramer

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D29176

llvm-svn: 293182