Make sure that stop-hooks run asynchronously.

They aren't designed to nest recursively, so this will prevent that.
Also add a --auto-continue flag, putting "continue" in the stop hook makes
the stop hooks fight one another in multi-threaded programs.
Also allow more than one -o options so you can make more complex stop hooks w/o
having to go into the editor.

<rdar://problem/48115661>

Differential Revision: https://reviews.llvm.org/D58394

llvm-svn: 354706