[ADT] Add a sum type abstraction for pointer-like types.

Description

[ADT] Add a sum type abstraction for pointer-like types.

This is a much more general and powerful form of PointerUnion. It
provides a reasonably complete sum type (from type theory) for
pointer-like types. It has several significant advantages over the
existing PointerUnion infrastructure:

  1. It allows more than two pointer types to participate without awkward nesting structures.
  2. It directly exposes the tag so that it is convenient to write switches over the possible members.
  3. It can re-use the same type for multiple tag values, something that has been worked around by either abusing PointerIntPair or defining nonce types and doing unsafe pointer casting.
  4. It supports customization of the PointerLikeTypeTraits used for specific member types. This means it could (in theory) be used even with types that are over-aligned on allocation to expose larger numbers of bits to the tag.

All in all, I think it is at least complimentary to the existing
infrastructure, and a strict improvement for some use cases.

Differential Revision: http://reviews.llvm.org/D15843