[Ada] Fold Enum_Rep attribute in evaluation and not in expansion

[Ada] Fold Enum_Rep attribute in evaluation and not in expansion

2020-06-10  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* exp_attr.adb (Expand_N_Attribute_Reference): Remove folding
	for Enum_Rep attribute.
	* exp_spark.adb (Expand_SPARK_N_Attribute_Reference): Remove
	duplicated code for folding Enum_Rep attribute.
	* sem_attr.adb (Eval_Attribute): Relax condition for folding
	Enum_Rep attribute; previously dead code is now executed when
	the attribute prefix is an enumeration literal; refine type in
	processing of Enum_Val.