[mlir][ods] Improve integer signedness modelling

A previous commit added support for integer signedness in C++
IntegerType. This change introduces ODS definitions for
integer types and integer (element) attributes w.r.t. signedness.

This commit also updates various existing definitions' descriptions
to mention signless where suitable to make it more clear.

Positive and non-negative integer attributes are removed to avoid
the explosion of subclasses. Instead, one should use more atmoic
constraints together with Confined to model that. For example,
`Confined<..., [IntPositive]>`.

Differential Revision: https://reviews.llvm.org/D75610