[API] Extend the `SBThreadPlan` interface

Summary:
This patch extends the `SBThreadPlan` to allow retrieving of thread plans
for scripted steps.

Reviewers: labath, zturner, jingham

Reviewed By: jingham

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D53361

llvm-svn: 345247