[mlir][RFC] Refactor layout representation in MemRefType

The change is based on the proposal from the following discussion:
https://llvm.discourse.group/t/rfc-memreftype-affine-maps-list-vs-single-item/3968

* Introduce `MemRefLayoutAttr` interface to get `AffineMap` from an `Attribute`
  (`AffineMapAttr` implements this interface).
* Store layout as a single generic `MemRefLayoutAttr`.

This change removes the affine map composition feature and related API.
Actually, while the `MemRefType` itself supported it, almost none of the upstream
can work with more than 1 affine map in `MemRefType`.

The introduced `MemRefLayoutAttr` allows to re-implement this feature
in a more stable way - via separate attribute class.

Also the interface allows to use different layout representations rather than affine maps.
For example, the described "stride + offset" form, which is currently supported in ASM parser only,
can now be expressed as separate attribute.

Reviewed By: ftynse, bondhugula

Differential Revision: https://reviews.llvm.org/D111553