[Ada] Improvements to implementation of Ada_2020 attribute Reduce

[Ada] Improvements to implementation of Ada_2020 attribute Reduce

gcc/ada/

	* sem_attr.adb (Min_Max): Handle the case where attribute
	name (qualified by required type) appears as the reducer of a
	'Reduce attribute reference.
	(Resolve_Attribute) <Reduce>: Handle properly the presence of a
	procedure or an attribute reference Min/Max as a reducer.
	* exp_attr.adb (Expand_Attribute_Reference) <Reduce>: New
	subprogram Build_Stat, to construct the combining statement
	which appears in the generated loop for Reduce, and which is
	either a function call when the reducer is a function or an
	attribute, or a procedure call when reducer is an appropriate
	procedure.  BuilD_Stat is used both when the prefix of 'Reduce
	is a value sequence and when it is an object