[Ada] Improve 'Val implementation for some enumeration types

[Ada] Improve 'Val implementation for some enumeration types

2020-06-03  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* einfo.ads (Enum_Pos_To_Rep): Adjust description.
	* exp_attr.adb (Expand_N_Attribute_Reference) <Pred>:
	Reimplement in the case of an enumeration type with non-standard
	but contiguous representation.
	<Succ>: Likewise.
	<Val>: Likewise.
	* exp_ch3.adb (Expand_Freeze_Enumeration_Type): Count the
	literals in the first loop.  If the representation is
	contiguous, just build the index type of the array type and set
	Enum_Pos_To_Rep to it.