[Ada] Allow Big_Integer in loop and subprogram variants

[Ada] Allow Big_Integer in loop and subprogram variants

In SPARK loop and subprogram variants we now allow expressions of any
discrete type and of Ada.Numerics.Big_Numbers.Big_Integers.Big_Integer
type.

gcc/ada/

	* exp_prag.adb (Expand_Pragma_Loop_Variant,
	Expand_Pragma_Subprogram_Variant): Adapt call via
	Process_Variant to Make_Variant_Comparison.
	* exp_util.adb (Make_Variant_Comparison): Compare Big_Integer
	expressions with a function call and not an operator.
	* exp_util.ads (Make_Variant_Comparison): Add type parameter,
	which is needed because the Curr_Val and Old_Val expressions
	might not be yet decorated.
	* rtsfind.ads: (RTU_Id): Add support for Big_Integers and
	Big_Integers_Ghost.
	(RE_Id): Add support for Big_Integer and its ghost variant.
	(RE_Unit_Table): Add mapping from Big_Integer to Big_Integers;
	same for the ghost variants.
	* rtsfind.adb (Get_Unit_Name): Add support for Big_Numbers.
	* sem_prag.adb (Analyze_Pragma): Allow Big_Integer in pragma
	Loop_Variant.
	(Analyze_Variant): Allow Big_Integer in pragma
	Subprogram_Variant.