[Ada] Implement AI12-0109 (prohibit some "early" derivations)

[Ada] Implement AI12-0109 (prohibit some "early" derivations)

2019-12-13  Steve Baird  <baird@adacore.com>

gcc/ada/

	* einfo.ads: Correct comment for Derived_Type_Link to reflect
	that fact that this function is now used for more than just
	generation of warnings.
	* sem_ch3.adb (Build_Derived_Type): Do not call
	Set_Derived_Type_Link if the derived type and the parent type
	are in different compilation units. Such a derivation cannot be
	a problematic "early" derivation (identifying these is what the
	Derived_Type_Link attribute is used for) and we don't like
	inter-unit references that go in the opposite direction of
	semantic dependencies.
	* sem_ch13.adb (Is_Type_Related_Rep_Item): A new function,
	analogous to the existing function Is_Operational_Item.
	(Rep_Item_Too_Late): Generate a hard error (with same text as
	the warning that was previously being generated) if the
	AI12-0109 legality rule is violated.

From-SVN: r279355