[dwarf reader] Support reference types without explicit DW_AT_byte_size

[dwarf reader] Support reference types without explicit DW_AT_byte_size

On x86_64 at least, in the debug info emitted by Clang, reference
types don't necessarily have the DW_AT_byte_size property.  In that
case, assume the size of the pointer type is the address size of the
current translation unit, rather than giving up and not building the
type.

	* src/abg-dwarf-reader.cc (build_reference_type): If the type DIE
	has no DW_AT_byte_size, assume the type size is the translation
	unit's address size.
	* tests/data/test-read-dwarf/test9-pr18818-clang.so.abi: Adjust.
	* tests/data/test-read-dwarf/test12-pr18844.so.abi: Adjust.

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