Fix PointerIntPair to be enum class compatible.

Some parts of PointerIntPair assumed that the IntType of the pair was implicitly
convertible to intptr_t, which is not the case for enum class values. Add a
static_cast<intptr_t> to make these conversions explicit and allow
PointerIntPair to be used with an enum class IntType. While we're here, rename
some of the argument values so we don't have variables named "Int" floating
around.

llvm-svn: 179073