* abg-ir.h (class_decl::set_earlier_declaration): Move this
out-of-line and add an overload.
* src/abg-ir.cc (class_decl::set_earlier_declaration): Moved this
out-of-line here and add an overload.
* src/abg-reader.cc (build_class_decl): Really read the
'decl-of-decl-id' property. Do not make the definition use the id
of the declaration. Both have their id.
* src/abg-writer.cc (write_class_decl): Put a space before the
property "def-of-decl-id".
* tests/data/test-read-write/test18.xml: New test input.
* tests/test-read-write.cc: De-serialize this new input, serialize
it back and make sure both versions are identical.
