[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