[ADT] Add a remarkbly useful little helper routine to ArrayRef for
checking whether the ArrayRef is equal to an explicit list of arguments.

This is particularly easy to implement even without variadic templates
because ArrayRef happens to be homogeneously typed. As a consequence we
can use a "clever" wrapper type and default arguments to capture in
a single method many arguments as well as *how many* arguments the user
specified.

Thanks to Dave Blaikie for helping me pull together this little helper.
Suggestions for how to improve or generalize it are of course welcome.
I'll be using it immediately in my follow-up patch. =D