[Ada] Spurious discriminant check on bounded synchronized queue

[Ada] Spurious discriminant check on bounded synchronized queue

gcc/ada/

	* libgnat/a-cbsyqu.ads (Implementation): Provide a box
	initialization for the element array used internally to
	represent the queue, so that its components are properly
	initialized if the given element type has default
	initialization. Suppress warnings on the rest of the package in
	case the element type has no default or discriminant, because it
	is bound to be confusing to the user.