[Ada] Implement AI12-0030: Stream attribute availability

gcc/ada/

	* sem_util.ads, sem_util.adb: Declare and implement a new
	predicate, Derivation_Too_Early_To_Inherit.  This function
	indicates whether a given derived type fails to inherit a given
	streaming-related attribute from its parent type because the
	declaration of the derived type precedes the corresponding
	attribute_definition_clause of the parent.
	* exp_tss.adb (Find_Inherited_TSS): Call
	Derivation_Too_Early_To_Inherit instead of unconditionally
	assuming that a parent type's streaming attribute is available
	for inheritance by an immediate descendant type.
	* sem_attr.adb (Stream_Attribute_Available): Call
	Derivation_Too_Early_To_Inherit instead of unconditionally
	assuming that a parent type's streaming attribute is available
	for inheritance by an immediate descendant type.
	* exp_attr.adb (Default_Streaming_Unavailable): A new predicate;
	given a type, indicates whether predefined (as opposed to
	user-defined) streaming operations for the type should be
	implemented by raising Program_Error.
	(Expand_N_Attribute_Reference): For each of the 4
	streaming-related attributes (i.e., Read, Write, Input, Output),
	after determining that no user-defined implementation is
	available (including a Stream_Convert pragma), call
	Default_Streaming_Unavailable; if that call returns True, then
	implement the streaming operation as "raise Program_Error;".

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