Convert PointerUnion to a variadic template.

Summary:
Rather than duplicating code between PointerUnion, PointerUnion3, and
PointerUnion4 (and missing things from the latter cases, such as some of the
DenseMap support and operator==), convert PointerUnion to a variadic template
that can be used as a union of any number of pointers.

(This doesn't support PointerUnion<> right now. Adding a special case for that
would be possible, and perhaps even useful in some situations, but it doesn't
seem worthwhile until we have a concrete use case.)

Reviewers: dblaikie

Subscribers: dexonsmith, kristina, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62027

llvm-svn: 360962