[Ada] Clear confusion about subcomponents of atomic object

[Ada] Clear confusion about subcomponents of atomic object

2019-12-13  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* sem_util.ads (Is_Atomic_Object): Mention relevant RM clauses.
	* sem_util.adb (Is_Atomic_Object): For an indexed component,
	only look at the Has_Atomic_Components aspect of the prefix and
	do not recurse on it; for a selected component, do not look at
	the prefix.
	(Is_Atomic_Or_VFA_Object): Minor tweak.

From-SVN: r279363