Move FPL_FALLTHROUGH_INTENDED to fplbase.
Move FPL_FALLTHROUGH_INTENDED to fplbase.

This attribute is needed to suppress a Clang warning for switch statement fallthroughs.
PiperOrigin-RevId: 146532060
Change-Id: Ic4fcf4b8cc7303ef4570f1ccba38ef8be9d45715

google/corgi,google/corgi