Don't cache the public stop reason, since it can change as plan completion...

Don't cache the public stop reason, since it can change as plan completion gets processed.  That means GetStopReason needs to return a shared pointer, not a pointer to the thread's cached version.  Also allow the thread plans to get and set the thread private stop reason - that is usually more appropriate for the logic the thread plans need to do.

llvm-svn: 116892