Moved the code in ClangUserExpression that set up & ran the thread plan with...

Moved the code in ClangUserExpression that set up & ran the thread plan with timeouts, and restarting with all threads into a utility function in Process.  This required a bunch of renaming. 

Added a ThreadPlanCallUserExpression that differs from ThreadPlanCallFunction in that it holds onto a shared pointer to its ClangUserExpression so that can't go away before the thread plan is done using it.

Fixed the stop message when you hit a breakpoint while running a user expression so it is more obvious what has happened.

llvm-svn: 120386