ada: Fix bogus error with "=" operator on array of private unchecked union

ada: Fix bogus error with "=" operator on array of private unchecked union

The code is legal and, therefore, must be accepted by the compiler, but it
must raise Program_Error at run time due to operands not having inferable
discriminants and a warning be given at compile time (RM B.3.3(22-23)).

gcc/ada/

	* exp_ch4.adb (Expand_Array_Equality.Component_Equality): Copy the
	Comes_From_Source flag from the original test to the new one, and
	remove obsolete code dealing with unchecked unions.
	* sem_util.adb (Has_Inferable_Discriminants): Return False for an
	incomplete or private nominal subtype.