[mlir] Add Adaptor alias

Summary:
Add Adaptor alias alongside OperandAdaptor to make next renaming more
mechanical. OperandAdaptor's are no longer just about operands.
Considered OpAdaptor too, but then noticed we'd mostly end up with
XOp::OpAdaptor which seems redundant.

Differential Revision: https://reviews.llvm.org/D81741