ada: Fix spurious warning about redundant parentheses in range bound

Use the same logic for warning about redundant parentheses in lower and upper
bounds of a discrete range. This fixes a spurious warning that, if followed,
would render the code illegal.

gcc/ada/ChangeLog:

	* par-ch3.adb (P_Discrete_Range): Detect redundant parentheses in the
	lower bound like in the upper bound.