Only add 'day' once with SqlServer DATEDIFF

If a single instance of a DATEDIFF Expression was used more than once, 'day' would be added multiple times (Once be usage). This threw an SQLServer error.

A simplistic example:
```
$age = $query->func()->dateDiff([
			'Jobs.date_queued' => 'literal',
			$functionBuilder->now('date'),
		]);
		$between30And60 = $query->newExpr()
			->lte(30, $age)
			->gt(60, $age);
```

This PR, in conjunction with PR #4200 allows for this use case.