[mlir][AttrDefGen] Add support for specifying the value type of an attribute

The value type of the attribute can be specified by either overriding the typeBuilder field on the AttrDef, or by providing a parameter of type `AttributeSelfTypeParameter`. This removes the need to define custom storage class constructors for attributes that have a value type other than NoneType.

Differential Revision: https://reviews.llvm.org/D97590