Add --move-to-nearest-code / target.move-to-nearest-code options (attempt 2)

This patch initially was committed in r237460 but later it was reverted (r237479) due to 4 new failures:
* TestExitDuringStep.py
* TestNumThreads.py
* TestThreadExit.py
* TestThreadStates.py

This patch also fixes these tests.

llvm-svn: 237566