Add declaration-only enums to XML reader/writer.

Add declaration-only enums to XML reader/writer.

	* src/abg-reader.cc (build_enum_type_decl): Detect a
	declaration-only enum and flag it as such.
	(build_type_decl): Support reading the "is-declaration" attribute.
	(build_class_decl): Adjust.
	* src/abg-writer.cc (write_is_declaration_only): Renamed
	write_class_or_union_is_declaration_only into this.
	(write_enum_is_declaration_only): Remove.
	(write_type_decl, write_enum_type_decl)
	(write_class_decl_opening_tag, write_union_decl_opening_tag): Use
	write_is_declaration_only.
	* tests/data/test-read-dwarf/PR22122-libftdc.so.abi: Adjust.
	* tests/data/test-read-dwarf/test12-pr18844.so.abi: Likewise.
	* tests/data/test-read-dwarf/test9-pr18818-clang.so.abi: Likewise.

Signed-off-by: Giuliano Procida <gprocida@google.com>
Signed-off-by: Dodji Seketeli <dodji@redhat.com>