ADT: Avoid using std::equal in ArrayRef::equals

MSVC's STL has a bug in `std::equal()`: it asserts on nullptr iterators,
causing a block revert in r215981.  This works around that by re-writing
`ArrayRef::equals()` to do the work itself.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215986 91177308-0d34-0410-b5e6-96231b3b80d8
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
CodingStandards: Document std::equal misbehaviour

I should have included this as part of r215986, which worked around this
corner by changing ArrayRef::equals() not to use std::equal.  Alas.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215988 91177308-0d34-0410-b5e6-96231b3b80d8