Model NamespaceAliasDecls as having their nominated namespace as an underlying

Model NamespaceAliasDecls as having their nominated namespace as an underlying
declaration. This fixes an issue where we would reject (due to a claimed
ambiguity) a case where lookup finds multiple NamespaceAliasDecls from
different scopes that nominate the same namespace.

The C++ standard doesn't make it clear that such a case is in fact valid (which
I'm working on fixing), but there are no relevant rules that distinguish using
declarations and namespace alias declarations here, so it makes sense to treat
them the same way.

llvm-svn: 256601