This PR adds promised interface declarations for
`ConvertToLLVMPatternInterface` in all the dialects that support the
`ConvertToLLVM` dialect extension.
Promised interfaces allow a dialect to declare that it will have an
implementation of a particular interface, crashing the program if one
isn't provided when the interface is used.
This patch introduces the `index` dialect and associated boilerplate for
adding ops and enums (comparison predicates).
Reviewed By: rriddle, jpienaar, nicolasvasilache
Differential Revision: https://reviews.llvm.org/D135688