Revamp our representation of C++ nested-name-specifiers. We now have a

uniqued representation that should both save some memory and make it
far easier to properly build canonical types for types involving
dependent nested-name-specifiers, e.g., "typename T::Nested::type".

This approach will greatly simplify the representation of
CXXScopeSpec. That'll be next.

llvm-svn: 67799