Don't use the third field of llvm.global_ctors for MachO.

The field is defined as:

If the third field is present, non-null, and points to a global variable or function, the initializer function will only run if the associated data from the current module is not discarded.

And without COMDATs we can't implement that.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218097 91177308-0d34-0410-b5e6-96231b3b80d8