[MLIR] Introduce constraint attributes for DenseArrayAttr

- `DenseArrayStrictlyPositive` all elements are required to be > 0.
  Returns true if the range is empty.

- `DenseArrayNonNegative` all elements are required to be >= 0. Returns
  true if the range is empty.

Both constraints will simplify verifier logic as we move from using `I64ArrayAttr` to `DenseI64ArrayAttr`.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D138988