[change-namespace] avoid adding leading '::' when possible.

Summary:
When changing namespaces, the tool adds leading "::" to references that need to
be fully-qualified, which would affect readability.

We avoid adding "::" when the symbol name does not conflict with the new
namespace name. For example, a symbol name "na::nb::X" conflicts with "ns::na"
since it would be resolved to "ns::na::nb::X" in the new namespace.

Reviewers: hokein

Reviewed By: hokein

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D30493

llvm-svn: 298363