ada: Fix bogus Constraint_Error on allocator for access to array of access type

This occurs because the access element type is not its own TYPE_CANONICAL,
which creates a discrepancy between the aliasing support code, which deals
with types directly, and the middle-end which looks at TYPE_CANONICAL only.

gcc/ada/

	* gcc-interface/decl.cc (gnat_to_gnu_entity) <E_Array_Type>: Use the
	TYPE_CANONICAL of types when it comes to aliasing.
	* gcc-interface/utils.cc (relate_alias_sets): Likewise.