This creates a couple of record subtypes pointing to each other through
access subtypes, and we break the circularity at the latter subtypes.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* gcc-interface/decl.cc (gnat_to_gnu_entity) <E_Record_Subtype>: If
	it is a special subtype designated by an access subtype, then defer
	the completion of incomplete types.