Fix PR4834, a tricky case where the inliner would resolve an

indirect function pointer, inline it, then go to delete the body.
The problem is that the callgraph had other references to the function,
though the inliner had no way to know it, so we got a dangling pointer
and an invalid iterator out of the deal.

The fix to this is pretty simple: stop the inliner from deleting the
function by knowing that there are references to it.  Do this by making
CallGraphNodes contain a refcount.  This requires moving deletion of 
available_externally functions to the module-level cleanup sweep where
it belongs.

llvm-svn: 80533