[analyzer] Treat @throw as a sink (stop processing).

The CFG approximates @throw as a return statement, but that's not good
enough in inlined functions. Moreover, since Objective-C exceptions are
usually considered fatal, we should be suppressing leak warnings like we
do for calls to noreturn functions (like abort()).

The comments indicate that we were probably intending to do this all along;
it may have been inadvertantly changed during a refactor at one point.

llvm-svn: 162156