[lldb] Add step target to ThreadPlanStepInRange constructor
QueueThreadPlanForStepInRange accepts a step_into_target, but the constructor for
ThreadPlanStepInRange does not. Instead, a caller would optionally call
SetStepInTarget() in a separate statement.
This change adds step_into_target as a constructor argument. This simplifies
construction of ThreadPlanSP, by avoiding a subsequent downcast and conditional
assignment. This constructor is already used in downstream repos.
Differential Revision: https://reviews.llvm.org/D96539