[mlir] Improve syntax of `distinct[n]<unit>`

In cases where memory is of less of a concern (e.g. small attributes where all instances have to be distinct by definition), using `DistinctAttr` with a unit attribute is a useful and conscious way of generating deterministic unique IDs.
The syntax as is however, makes them less useful to use, as it 1) always prints `<unit>` at the back and 2) always aliases them leading to not very useful `#distinct = distinct[n]<unit>` lines in the printer output.

This patch fixes that by special casing `UnitAttr` to simply elide the `unit` attribute in the back and not printing it as alias in that case.

Differential Revision: https://reviews.llvm.org/D155162