[Ada] Put_Image: Implement for private types with full real type

[Ada] Put_Image: Implement for private types with full real type

2020-06-15  Bob Duff  <duff@adacore.com>

gcc/ada/

	* exp_imgv.adb (Expand_Image_Attribute): Allow private types.
	Put_Image generates Image for numeric types, and private types
	whose full type is numeric. This requires the Conversion_OK flag
	for integer and floating-point types. For fixed point, we need
	the extra conversion.
	* exp_put_image.adb (Build_Elementary_Put_Image_Call): Remove
	special handling of real types.
	(Enable_Put_Image): Enable for reals.