Consider nested namespaces in the canonical namespace as canonical as well.

Consider nested namespaces in the canonical namespace as canonical as well.

Summary:
For example, this case was missed when looking for different but canonical
namespaces. UseContext in this case should be considered as in the canonical
namespace.
```
namespace a { namespace b { <FromContext> } }
namespace a { namespace b { namespace c { <UseContext> } } }
```
Added some commenting.

Reviewers: bkramer

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D27125

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@287924 91177308-0d34-0410-b5e6-96231b3b80d8