When the prefix of an Access attribute is an explicit dereference of an
access parameter (or a renaming of such a dereference, or a subcomponent
of such a dereference), the context is a general access type to a
class-wide interface type, and an accessibility check must be generated,
the frontend silently skips generating an implicit type conversion to
force the displacement of the pointer to reference the secondary
dispatch table.

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

gcc/ada/

	* exp_attr.adb (Add_Implicit_Interface_Type_Conversion): New
	subprogram which factorizes code.
	(Expand_N_Attribute_Reference): Call the new subprogram to add
	the missing implicit interface type conversion.