[Ada] Implement AI12-0369

[Ada] Implement AI12-0369

2020-06-12  Steve Baird  <baird@adacore.com>

gcc/ada/

	* sem_util.ads, sem_util.adb: Define 3 new Boolean-valued
	functions - Statically_Denotes_Entity,
	Statically_Denotes_Object, and Statically_Names_Object. The
	first two were taken from sem_attr.adb. The term "statically
	names" is defined in the Ada RM and the new function
	Statically_Names_Object is intended to reflect that definition,
	or more precisely, as described in a comment in the code, to
	reflect the expected future definition of that term.
	* sem_attr.adb: Delete functions Statically_Denotes_Object and
	Statically_Denotes_Entity; these two functions have been moved
	to package Sem_Util. Replace call to Statically_Denotes_Object
	with a call to Statically_Names_Object as per AI12-0217 (a
	binding interpretation, so no Ada_Version check).
	* exp_ch9.adb (Expand_Entry_Barrier.Is_Simple_Barrier): Change
	name of function (it was previously Is_Simple_Barrier_Name)
	because the function should return True in the case of a static
	expression; implement this requirement.  Change function to
	include a call to Statically_Names_Object so that, for Ada_2020
	and later, it will return True for appropriate subcomponent
	names.
	(Expand_Entry_Barrier.Is_Pure_Barrier): Handle
	N_Indexed_Component and N_Selected_Component cases by calling
	Statically_Names_Object.
	(Expand_Entry_Barrier): Reorganize to treat Simple_Barriers and
	Pure_Barriers more uniformly.  Prevent cascaded errors.