Make ThreadPlanStepInstruction's constructor public.

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.

Reviewers: clayborg, zturner

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D22230

llvm-svn: 275139