DebugInfo/DWARF: Refactor type dumping to dump types, rather than DIEs that reference types

This lays the foundation for dumping types not referenced by DW_AT_type
attributes (in the near-term, that'll be DW_AT_containing_type for a
DW_TAG_ptr_to_member_type - in the future, potentially dumping the
pretty printed name next to the DW_TAG for the type, rather than only
when the type is referenced from elsewhere)

llvm-svn: 348961