[Ada] Cleanup condition for an effectively volatile array type

[Pierre-Marie de Rodat]

When we detect effectively volatile array type we only need to examine
the type of array component if the array itself has no
Has_Volatile_Components property.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* sem_prag.adb (Atomic_Components): Simplify with Ekind_In.
	(Complex_Representation): Fix type of E_Id, which just like when
	for pragma Atomic_Components will hold an N_Identifier node, not
	an entity.
	* sem_util.adb (Is_Effectively_Volatile): Refactor to avoid
	unnecessary computation.
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