handle implicit fallthrough in switch statement
handle implicit fallthrough in switch statement

as discussed in #452 break statements and a fallthrough comment was
added to handle -Werror=implicit-fallthrough for gcc7.

p4lang/behavioral-model,p4lang/behavioral-model,hanw/behavioral-model,hanw/behavioral-model,hanw/behavioral-model,hanw/behavioral-model,p4lang/behavioral-model,p4lang/behavioral-model,p4lang/behavioral-model,hanw/behavioral-model