Reapply r215966, r215965, r215964, r215963, r215960, r215959, r215958, and r215957

Reapply r215966, r215965, r215964, r215963, r215960, r215959, r215958, and r215957

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