[Target] Generalize language-specific behavior in ThreadPlanStepThrough

Summary:
When creating a ThreadPlan to step through a trampoline, we ask the
ObjC language runtime and the CPP language runtime to come up with such a thread
plan if the dynamic loader fails to give us one. I don't see why this behavior
can't be language agnostic.

Differential Revision: https://reviews.llvm.org/D61921

llvm-svn: 362164