[Ada] Missing front-end code for constraint checks on fixed point exprs

[Ada] Missing front-end code for constraint checks on fixed point exprs

This patch ensures that the front-end generates constraint checks for
some operations that previously depended on gigi for the corresponding
check. The patch also resets the Do_Range_Check flag so that it never
appears in the tree presented to gigi.

2018-09-26  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* checks.adb (Apply_Type_Conversion_Checks): Do not generate a
	range check on the expression of the conversion if it involves a
	fixed-point type, as such conversions are handled specially
	during expansion.
	* exp_ch4.adb (Expand_N_Type_Conversion): In a conversion from
	Fixed to Integer, use the base type of the expression to ensure
	that the caller will generate the proper constraint check when
	needed.

From-SVN: r264632