Teach NestedNameSpecifier to keep track of namespace aliases the same

Teach NestedNameSpecifier to keep track of namespace aliases the same
way it keeps track of namespaces. Previously, we would map from the
namespace alias to its underlying namespace when building a
nested-name-specifier, losing source information in the process.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@126358 91177308-0d34-0410-b5e6-96231b3b80d8