ada: Allow passing private types to generic formal incomplete types

ada: Allow passing private types to generic formal incomplete types

It is legal to pass a private type, or a type with a component whose
type is private, as a generic actual type if the formal is a generic
formal incomplete type. This patch fixes a bug in which the compiler
would give an error in some such cases.

Also misc cleanup.

gcc/ada/

	* sem_ch12.adb (Instantiate_Type): Make the relevant error message
	conditional upon "Ekind (A_Gen_T) /= E_Incomplete_Type". Misc
	cleanup.