PR15452 Follow DW_AT_signature attributes when trying to find a DW_AT_type.

PR15452 Follow DW_AT_signature attributes when trying to find a DW_AT_type.

When we are trying to find a type DIE we might hit upon a DIE that simply
has a DW_AT_signature pointing to the actual type (possibly in .debug_type).
Extend the debugtypes.exp testcase a bit. Add DW_AT_signature chasing to
dwarf_wrappers.h (dwarf_attr_die) when searching for a DW_AT_type. And
clarify that dwflpp::print_members takes a type DIE not a variable DIE.