[Ada] Fix missing Constraint_Error for Enum_Val attribute

[Ada] Fix missing Constraint_Error for Enum_Val attribute

This fixes an old issue involving the Enum_Val attribute: it does not
always raise a Constraint_Error exception when the specified value is
not valid for the enumeration type (instead a modulo computation is
applied to the value).

2019-07-22  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* exp_attr.adb (Expand_N_Attribute_Reference)
	<Attribute_Enum_Val>: Set No_Truncation on the
	N_Unchecked_Type_Conversion built around the argument passed to
	the attribute.

gcc/testsuite/

	* gnat.dg/enum_val1.adb: New testcase.

From-SVN: r273676