Reflection: Share demangler with MetadataReader.
TypeRefBuilder and MetadataReader had nearly identical symbolic reference resolvers,
but diverged because TypeRefBuilder had its own local/remote address management mechanism,
and because TypeRefBuilder tries to resolve opaque types to their underlying types, whereas
other MetadataReader clients want to preserve them as written in source. The first problem
has been addressed by making TypeRefBuilder use `RemoteRef` everywhere, and the second
can be handled with a flag (and might be able to be handled more elegantly with some more
refactoring of general opaque type handling in MetadataReader).
