Add TinyPtrVector support for general pointer-like things.

In particular, make TinyPtrVector<PtrIntPair<T *, 1>> work. Remove all
unnecessary assumptions that the element type has a formal "null"
representation. The important property to maintain is that
default-constructed element type has the same internal representation
as the default-constructed PointerUnion (all zero bits).

Remove the incorrect recursive behavior from
PointerUnion::isNull. This was never generally correct because it only
recursed over the first type parameter. With variadic templates it's
completely unnecessary.

llvm-svn: 369473