Consider DW_TAG_ptr_to_member_type a pointer when decoding attribute values

- The test-case is a hacked-up version of assembly-dump of
  pointer_const_value.c.  It might be illagal or nonsensical Dwarf, I
  didn't check.
Do not bail out on non-DW_TAG_compile_unit CU's

- This is a far cry from a decent support of partial units--merge
  needs to be implemented, and there are some attribute forms in this
  area that we don't support at all (the DW_FORM_GNU_ref_{alt,strp}
  stuff in particular).  But it's a first step.