Make the COFF backend linker merge common types:

	Make the COFF backend linker merge common types:
	* cofflink.c (struct coff_debug_merge_element): Define.
	(struct coff_debug_merge_type): Define.
	(struct coff_debug_merge_hash_entry): Define.
	(struct coff_debug_merge_hash_table): Define.
	(coff_debug_merge_hash_table_init): Define.
	(coff_debug_merge_hash_table_free): Define.
	(coff_debug_merge_hash_lookup): Define.
	(struct coff_final_link_info): Add debug_merge field.
	(coff_debug_merge_hash_newfunc): New static function.
	(_bfd_coff_final_link): Allocate and free debug_merge table.
	(coff_link_input_bfd): Merge identical enum, struct and union
	types.
PR 7145.