[mlir] Make DenseArrayAttr generic

This patch turns `DenseArrayBaseAttr` into a fully-functional attribute by
adding a generic parser and printer, supporting bool or integer and floating
point element types with bitwidths divisible by 8. It has been renamed
to `DenseArrayAttr`. The patch maintains the specialized subclasses,
e.g. `DenseI32ArrayAttr`, which remain the preferred API for accessing
elements in C++.

This allows `DenseArrayAttr` to hold signed and unsigned integer elements:

```
array<si8: -128, 127>
array<ui8: 255>
```

"Exotic" floating point elements:

```
array<bf16: 1.2, 3.4>
```

And integers of other bitwidths:

```
array<i24: 8388607>
```

Reviewed By: rriddle, lattner

Differential Revision: https://reviews.llvm.org/D132758