Add a mechanism for Dialects to provide a fallback for OpInterface

This mechanism makes it possible for a dialect to not register all
operations but still answer interface-based queries.
This can useful for dialects that are "open" or connected to an external
system and still interoperate with the compiler. It can also open up the
possibility to have a more extensible compiler at runtime: the compiler
does not need a pre-registration for each operation and the dialect can
inject behavior dynamically.

Reviewed By: rriddle, jpienaar

Differential Revision: https://reviews.llvm.org/D93085