Make ThreadPlanStepInstruction's constructor public.

[sas]

Summary

Some thread plans have public contructors, some others have protected
constructors with friend classes. Not sure how these were determined,
but this thread plan is going to be required to implement trampoline
step-through on Windows.

[clayborg]

Looks good to me, but for any thread plan stuff, I will defer to Jim Ingham.

[jingham]

Sure, yes.

[sas]

Rebase.

include/lldb/Target/ThreadPlanStepInstruction.h

 #include "lldb/Target/Thread.h"
 #include "lldb/Target/ThreadPlan.h"
 
 namespace lldb_private {
 
 class ThreadPlanStepInstruction : public ThreadPlan
 {
 public:
+    ThreadPlanStepInstruction (Thread &thread,
+                               bool step_over,
+                               bool stop_others,
+                               Vote stop_vote,
+                               Vote run_vote);
+
     ~ThreadPlanStepInstruction() override;
 
     void GetDescription(Stream *s, lldb::DescriptionLevel level) override;
     bool ValidatePlan(Stream *error) override;
     bool ShouldStop(Event *event_ptr) override;
     bool StopOthers() override;
     lldb::StateType GetPlanRunState() override;
     bool WillStop() override;
     bool MischiefManaged() override;
     bool IsPlanStale() override;
 
 protected:
     bool DoPlanExplainsStop(Event *event_ptr) override;
 
-    ThreadPlanStepInstruction (Thread &thread,
-                               bool step_over,
-                               bool stop_others,
-                               Vote stop_vote,
-                               Vote run_vote);
     void SetUpState ();
 
 private:
     friend lldb::ThreadPlanSP
     Thread::QueueThreadPlanForStepSingleInstruction (bool step_over, bool abort_other_plans, bool stop_other_threads);
 
     lldb::addr_t m_instruction_addr;
     bool m_stop_other_threads;