Ada: Fix wrong Accum_Type inferred for a reduction expression

Ada: Fix wrong Accum_Type inferred for a reduction expression

This was reported as a regression in GCC 14: the compiler resolves
Accum_Type to Positive for a reduction expression whose "expected
subtype" is Positive, which means that 0 cannot be used as initial
value in the expression:

  Sum : Positive := V'Reduce ("+", 0);

without always raising Constraint_Error as run time.  That's not the
intent according to T. Taft in
  https://forum.ada-lang.io/t/regression-in-gnat-14/890
so this changes the resolution to use the base type (Integer) instead.

gcc/ada/
	PR ada/115349
	* sem_attr.adb (Resolve_Attribute) <Attribute_Reduce>: Use the base
	type as Accum_Type if the reducer is an operator from Standard and
	the type is numeric.  Use the type of the first operand for other
	operators.  Streamline the error message given for limited types.

gcc/testsuite/
	* gnat.dg/reduce3.adb: New test.