dwarf-reader: properly set artificial-ness in opaque types

dwarf-reader: properly set artificial-ness in opaque types

get_opaque_version_of_type forgets to set the "is-artificial" property
according to the initial type the opaque type is derived from.  This
can lead to some instability in the abixml output.

Fixed thus.

	* src/abg-dwarf-reader.cc (get_opaque_version_of_type): Propagate
	the artificial-ness of the original type here.
	* tests/data/test-read-dwarf/PR27700/test-PR27700.abi: Adjust.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>