* sem_case.adb (Check_Choices): Add explanatory message when there are
missing alternatives when the required range of alternatives is given
by the base type of the case expression or discriminant in a variant
part.
* opt.ads: New flag Warn_On_Overlap, to enable warnings on potentially
dangerous overlap between actuals in a call, activated by -gnatw.i
* sem_warn.adb (Set_Dot_Warning_Switch): set flag.
(Warn_On_Overlapping_Actuals): use new flag.
* gnat_ugn.texi: Document -gnatw.i, warning on overlapping actuals