GlobalOpt: fix an issue where CXAAtExitFn points to a deleted function.

CXAAtExitFn was set outside a loop and before optimizations where functions
can be deleted. This patch will set CXAAtExitFn inside the loop and after
optimizations.

Seg fault when running LTO because of accesses to a deleted function.
rdar://problem/13838828


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181838 91177308-0d34-0410-b5e6-96231b3b80d8