[Ada] Expand 'Pos and 'Val for enumeration types with standard representation

[Ada] Expand 'Pos and 'Val for enumeration types with standard representation

2020-06-16  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* sinfo.ads (Conversion_OK): Document use for 'Pos and 'Val.
	* exp_attr.adb (Get_Integer_Type): New function returning a
	small integer type appropriate for an enumeration type.
	(Expand_N_Attribute_Reference) <Attribute_Enum_Rep>: Call it.
	<Attribute_Pos>: For an enumeration type with a standard
	representation, expand to a conversion with Conversion_OK.
	<Attribute_Val>: Likewise.
	* exp_ch4.adb (Expand_N_Type_Conversion): Do not expand when
	the target is an enumeration type and Conversion_OK is set.