[mlir][tblgen] Generate generic adaptors for Ops

[mlir][tblgen] Generate generic adaptors for Ops

This is part of the RFC for a better fold API: https://discourse.llvm.org/t/rfc-a-better-fold-api-using-more-generic-adaptors/67374

This patch implements the generation of generic adaptors through TableGen. These are essentially a generalization of Adaptors, as implemented previously, but instead of indexing into a `mlir::ValueRange`, they may index into any container, regardless of the element type. This allows the use of the convenient getter methods of Adaptors to be reused on ranges that are the result of some kind of mapping functions of an ops operands.
In the case of the fold API in the RFC, this would be `ArrayRef<Attribute>`, which is a mapping of the operands to their possibly-constant values.

Implementation wise, some special care was taken to not cause a compile time regression, nor to break any kind of source compatibility.
For that purpose, the current adaptor class was split into three:
* A generic adaptor base class, within the detail namespace as it is an implementation detail, which implements all APIs independent of the range type used for the operands. This is all the attribute and region related code. Since it is not templated, its implementation does not have to be inline and can be put into the cpp source file
* The actual generic adaptor, which has a template parameter for the range that should be indexed into for retrieving operands. It implements all the getters for operands, as they are dependent on the range type. It publicly inherits from the generic adaptor base class
* A class named as adaptors have been named so far, inheriting from the generic adaptor class with `mlir::ValueRange` as range to index into. It implements the rest of the API, specific to `mlir::ValueRange` adaptors, which have previously been part of the adaptor. This boils down to a constructor from the Op type as well as the verify function.

The last class having the exact same API surface and name as Adaptors did previously leads to full source compatibility.

Differential Revision: https://reviews.llvm.org/D140660