Use dwarf2_full_name when computing type names

This changes a few spots in the DWARF reader to use dwarf2_full_name
when computing the name of a type.  This gives the correct name when a
type is nested in a namespace.  This oddity probably wasn't noticed
before because some of the types in question are either normally
anonymous in C++ (e.g, array type) or do not appear in a namespace
(base type).