ada: Fix spurious range check for out mode scalars in class-wide preconditions

This patch fixes a spurious range check that was generated for out mode scalars
after a class-wide precondition call. This issue was not present in static
preconditions, but only in dynamic ones. This patch refactors the two helper
call builders into the same to ensure the same processing of actuals.

gcc/ada/ChangeLog:

	* exp_ch6.adb (Build_Helper_Call): Refactor code to handle both static and
	dynamic precondition calls in the same way.