Add pretty printers for llvm::PointerIntPair and llvm::PointerUnion.

Reviewers: aprantl, dblaikie, jdoerfert, nicolasvasilache

Reviewed By: dblaikie

Subscribers: jpienaar, dexonsmith, merge_guards_bot, llvm-commits

Tags: #llvm, #clang, #lldb, #openmp

Differential Revision: https://reviews.llvm.org/D72557