swift234

Replace uses of llvm::Fixnum with llvm::PointerEmbeddedInt.

The two types are nearly identical, and Fixnum is only in the Swift branches of LLVM,
not in mainline LLVM.

I do want to add ++ to PointerEmbeddedInt and fix some of this ugliness, but that'll
have to go through LLVM review, so it might take a bit.