Use canonical types hash maps for type IDs in abixml writer

Use canonical types hash maps for type IDs in abixml writer

This patch stores canonical types in the hash maps that are used in
the abixml writer to compute the type IDs.  This limits the
possibility that two types that are equivalent (especially when one is
a declaration-only class) end-up in the same abixml.

	* src/abg-writer.cc (write_context::{type_has_existing_id,
	get_id_for_type}): Save the canonical type of the type in the map,
	not the type itself.
	(write_context::{type_is_emitted}): Use the canonical type rather
	than the type itself.
	* tests/data/test-read-dwarf/test9-pr18818-clang.so.abi: Adjust.

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