Fix the process of getting the ObjC runtime - if we ask for it too early (in...

Fix the process of getting the ObjC runtime - if we ask for it too early (in the process of handling the
load notification for the first load) then we will set it the runtime to NULL and won't re-search for it.
Added a way for the dynamic loader to force a re-search, since it knows the world has changed.

llvm-svn: 152453