This reverts commit r215981, which reverted the above commits because
MSVC std::equal asserts on nullptr iterators, and thes commits
introduced an `ArrayRef::equals()` on empty ArrayRefs.
ArrayRef was changed not to use std::equal in r215986.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215987 91177308-0d34-0410-b5e6-96231b3b80d8