[Ada] Convert operands of generated code to Standard.Unsigned for modular ops.

[Ada] Convert operands of generated code to Standard.Unsigned for modular ops.

2018-05-31  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* exp_ch4.adb (Expand_Modular_Addition, Expand_Modular_Subtraction):
	Convert all operands of the generated code to Standard.Unsigned, to
	prevent spurious visibility errors. This is needed when the generated
	expansion involves a modular index type of an array type, and the index
	type is not immediately visible.

From-SVN: r260993