ada: Error in determining accumulator subtype for a reduction expression

There was an earlier bug in determining the accumulator subtype for a
reduction expression in the case where the reducer subprogram is overloaded.
The fix for that bug introduced a recently-discovered
regression. Redo accumulator subtype computation in order to address
this regression while preserving the benefits of the earlier fix.

gcc/ada/

	* exp_attr.adb:	Move computation of Accum_Typ entirely into the
	function Build_Stat.