[mlir][OpAsmFormat] Add support for an "else" group on optional elements

The "else" group of an optional element is a collection of elements that get parsed/printed when the anchor of the main element group is *not* present. This is useful when there is a special syntax when an element is not present. The new syntax for an optional element is shown below:

```
optional-group: `(` elements `)` (`:` `(` else-elements `)`)? `?`
```

An example of how this might be used is shown below:

```tablegen
def FooOp : ... {
  let arguments = (ins UnitAttr:$foo);

  let assemblyFormat = "attr-dict (`foo_is_present` $foo^):(`foo_is_absent`)?";
}
```

would be formatted as such:

```mlir
// When the `foo` attribute is present:
foo.op foo_is_present

// When the `foo` attribute is not present:
foo.op foo_is_absent
```

Differential Revision: https://reviews.llvm.org/D99129