[ptr-traits] Move a class definition up to the top of this header so it

can be referenced as part of a PointerIntPair.

This is part of a series of patches to allow LLVM to check for complete
pointee types when computing its pointer traits. This is absolutely
necessary to get correct (or reproducible) results for things like how
many low bits are guaranteed to be zero.

llvm-svn: 256613