Fix PointerIntPair so that it can use an enum class as its integer template argument.

Summary:
The problem here is that an enum class can not be implicitly converted to an
integer. That assumption snuck back into PointerIntPair. This commit fixes the
issue and more importantly adds some unittests to make sure that we do not break
this again.

rdar://23594806

Reviewers: gribozavr

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D16131

llvm-svn: 257574