Add libclang support for namespace aliases (visitation + USRs) along

Add libclang support for namespace aliases (visitation + USRs) along
with a new cursor kind for a reference to a namespace.

There's still some oddities in the source location information for
NamespaceAliasDecl that I'll address with a separate commit, so the
source locations displayed in the load-namespaces.cpp test will
change.

llvm-svn: 112676