[lldb] Add deleted line in NextRangeBreakpointExplainsStop

Commit f838fa8 refactored large
pointers of the thread plan code. In that refactor, the call to
ClearNextBranchBreakpoint in
ThreadPlanStepRange::NextRangeBreakpointExplainsStop was deleted, I
believe by mistake.

This patch simply re-adds that call.

rdar://159675204

Author: augusto2112

lldb/source/Target/ThreadPlanStepRange.cpp

@@ -487,6 +487,7 @@ bool ThreadPlanStepRange::NextRangeBreakpointExplainsStop(
             "next range breakpoint which has %" PRIu64
             " constituents - explains stop: %u.",
             (uint64_t)num_constituents, explains_stop);
+  ClearNextBranchBreakpoint();
   return explains_stop;
 }