Some fixes for thread stepping on Windows.

This hooks up the changes necessary to set the trap flag on the
CPU and properly manage the process and thread's resume state
and private state so that the ThreadPlan does its thing.

Stepping still doesn't work as of this change, because there are
some issues with stack frames where it doesn't update the thread's
frame list correctly when it breaks inside of a function, but
I will try to fix that separately.

llvm-svn: 226221