[Function] Properly remove use when clearing personality

Summary:
We need to actually remove the use of the personality function,
otherwise we can run into trouble if we want to e.g. delete
the personality function because ther's no way to get rid of
its uses. Do this by resetting to ConstantPointerNull value
that the operands are set to when first allocated.

Reviewers: vsk, dexonsmith

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D15752

llvm-svn: 256345