ada: Use accumulator type in expansion of 'Reduce attribute

ada: Use accumulator type in expansion of 'Reduce attribute

The current expansion of the 'Reduce attribute uses the resolution type of
the expression for the accumulator. Now this type can be unresolved or set
to a universal type, for example if it is itself the prefix of the 'Image
attribute, and this may yield a spurious type mismatch error in that case.

This changes the expansion to use the accumulator type instead as defined
by the RM 4.5.10 clause, albeit only in the prefixed case for now.

gcc/ada/

	* exp_attr.adb (Expand_N_Attribute_Reference) <Attribute_Reduce>:
	Use the canonical accumulator type as the type of the accumulator
	in the prefixed case.