ada: Reference to nonexistent operator in reduction expression accepted

ada: Reference to nonexistent operator in reduction expression accepted

In some cases, a reduction expression that references the (nonexistent)
"+" operator of a generic formal private type is incorrectly accepted.

gcc/ada/

	* sem_attr.adb (Resolve_Attribute.Proper_Op): When resolving the
	name of the reducer subprogram in a reduction expression,
	Proper_Op treats references to operators defined in Standard
	specially. Disable this special treatment if the type of the
	reduction expression is not the right class of type for the
	operator, or if a new Boolean parameter (named "Strict") is True.
	(Resolve_Attribute): In the overloaded case, iterate over the
	reducer subprogram candidates twice. First with Strict => True and
	then, if no good intepretation is found, with Strict => False.