[Ada] Uintp: add a new UI_From_Integral generic constructor

[Ada] Uintp: add a new UI_From_Integral generic constructor

2018-12-11  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* uintp.ads, uintp.adb (UI_From_Integral): New generic function,
	to simplify construction of Universal_Integer representations
	from any Integer type. If type is small enough the code is
	equivalent to a call to UI_To_Int with appropriate conversions;
	otherwise the routine uses the existing mechanism of building a
	vector of suitable integer values and calling Vector_To_Uint.
	The routine must not be applied to a biased type.

From-SVN: r267000