swift234

[serialization] Replace hand-rolled PointerUnion with simple wrapper.

I avoided this the first time around because I wasn't sure if Type really had
low bits free, but it looks like we're relying on that elsewhere. If we ever
use up all the bits in Type we'll have to rip all these up, but for now it's
better to stick with what's in LLVM.

Swift SVN r5322