ada: Improved support for incomplete parameter types

ada: Improved support for incomplete parameter types

Fix two bugs uncovered by a recent ACATS test C3A1005: a freezing problem
and a case where a user-defined equality function for an incomplete type
was incorrectly hidden from use-clause visibility by the "corresponding"
predefined op (which doesn't actually exist).

gcc/ada/ChangeLog:

	* sem_ch6.adb (Analyze_Subprogram_Body_Helper): Don't freeze here
	if Has_Delayed_Freeze returns True.
	* sem_type.adb (Valid_Equality_Arg): Treat an incomplete type like
	a limited type because neither has an implicitly-defined equality
	primitive.
	(Covers): If either argument is an incomplete type
	whose full view is available, then look through to the full view.
	* sem_res.adb (Resolve_Actuals): If the actual parameter type is
	complete and the formal parameter type is not, then update the
	formal parameter type to use the complete view.