[Ada] Implement new legality checks specified by AI12-0412

[Ada] Implement new legality checks specified by AI12-0412

gcc/ada/

	* freeze.adb (Check_Inherited_Conditions): Setting of Ekind,
	LSP_Subprogram, and Is_Wrapper needs to happen for null
	procedures as well as other wrapper cases, so the code is moved
	from the else part in front of the if statement.  (Fixes a
	latent bug encountered while working on this set of changes.)
	* sem_attr.adb (Resolve_Attribute): Report an error for the case
	of an Access attribute applied to a primitive of an abstract
	type when the primitive has any nonstatic Pre'Class or
	Post'Class expressions.
	* sem_ch8.adb (Analyze_Subprogram_Renaming): Report an error for
	the case of a actual subprogram associated with a nonabstract
	formal subprogram when the actual is a primitive of an abstract
	type and the primitive has any nonstatic Pre'Class or Post'Class
	expressions.
	* sem_disp.adb (Check_Dispatching_Context): Remove special
	testing for null procedures, and replace it with a relaxed test
	that avoids getting an error about illegal calls to abstract
	subprograms in cases where RM 6.1.1(7/5) applies in
	Pre/Post'Class aspects. Also, remove special test for
	Postcondition, which seems to be unnecessary, update associated
	comments, and fix a typo in one comment.
	(Check_Dispatching_Call): Remove an unneeded return statement,
	and report an error for the case of a nondispatching call to a
	nonabstract subprogram of an abstract type where the subprogram
	has nonstatic Pre/Post'Class aspects.
	* sem_util.ads
	(Is_Prim_Of_Abst_Type_With_Nonstatic_CW_Pre_Post): New function.
	(In_Pre_Post_Condition): Add a flag formal Class_Wide_Only,
	defaulted to False, for indicating whether the function should
	only test for the node being within class-wide pre- and
	postconditions.
	* sem_util.adb
	(Is_Prim_Of_Abst_Type_With_Nonstatic_CW_Pre_Post): New function
	to determine whether a subprogram is a primitive of an abstract
	type where the primitive has class-wide Pre/Post'Class aspects
	specified with nonstatic expressions.
	(In_Pre_Post_Condition): Extend testing to account for the new
	formal Class_Wide_Only.