[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