[ptr-traits] Move methods manipulating PointerUnions, DenseMap pointer

keys, and PointerIntPairs where the pointee types are incomplete
out-of-line to where we have the complete type.

This is the standard pattern used throughout the AST library to address
the inherently mutually cross referenced nature of the AST.

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: 256612