[ptr-traits] Add a bunch of includes to provide complete types that are

used in pointer dense map key types or in other ways that require
pointer traits.

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: 256549