[change-namespace] Enhance detection of conflicting namespaces.

Summary:
For example:
```
namespace util { class Base; }

namespace new {
namespace util { class Internal; }
}

namespace old {
util::Base b1;
}
```

When changing `old::` to `new::`, `util::` in namespace "new::" will conflict
with "new::util::" unless a leading "::" is added.

Reviewers: hokein

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D53489

llvm-svn: 344897