[Ada] Spurious errors on aspect specifications in generic units

[Ada] Spurious errors on aspect specifications in generic units

This patch fixes spurious errors on aspect specifications on record
types when the aspect expression references a component of the type that
is not a discriminant. The patch also cleans up the legality checks on
aspect specifications, and improves error message on illegal aspect
specifications whose expressions are not conformant between
specification and freeze point, because of changes in visibility.

2018-12-11  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_ch13.adb (Push_Type, Pop_Type): New procedures, used for
	analysis of aspect expressions for record types, whose
	components (not only discriminants) may be referenced in aspect
	expressions.
	(Analyze_Aspect_Specifications, Analyze_Aspects_At_Freeze_Point,
	Analyze_Aspect_At_End-Of_Declarations,
	Resolve_Aspect_Expressions): Use the new subprograms.
	(Check_Aspect_At_End_Of_Declarations): Improve error message.
	(Build_Predicate_Functions): Do not build their bodies in a
	generic unit.
	(Is_Derived_Type_With_Constraint): New subprogram to uncover and
	reject aspect specificationss on types that appear after the
	type is frozen.
	* sem_ch13.ads (Push_Scope_And_Install_Discriminants,
	Uninstall_Discriminants_And_Pop_Scope): Remove.
	* sem_ch6.adb, sem_ch6.ads (Fully_Conformant_Expressions):
	Additional parameter to improve error message on illegal aspect
	specifications whose resolution differ between aspect
	specification and freeze point.
	* freeze.adb: Remove references to
	Install/Uninstall_Discriminants.

gcc/testsuite/

	* gnat.dg/aspect1.adb, gnat.dg/aspect1_horizontal.adb,
	gnat.dg/aspect1_horizontal.ads, gnat.dg/aspect1_vectors_2d.ads:
	New testcase.
	* gnat.dg/static_pred1.adb: Expect an error message.

From-SVN: r266980