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