[Ada] Refine generation of range checks to happen in front end

[Ada] Refine generation of range checks to happen in front end

2018-07-31  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* exp_attr.adb (Expand_Attribute, case Fixed_Value): Set the
	base type of the result to ensure that proper overflow and range
	checks are generated.  If the target is a fixed-point tyoe,
	generate the required overflow and range checks explicitly,
	rather than relying on Apply_Type_Conversion_Checks, which might
	simply set the Do_Range_Check flag and rely on the backend to
	add the check.

From-SVN: r263104