[mlir][ods] Support string literals in `custom` directives

This patch adds support for string literals as `custom` directive
arguments. This can be useful for re-using custom parsers and printers
when arguments have a known value. For example:

```
ParseResult parseTypedAttr(AsmParser &parser, Attribute &attr, Type type) {
  return parser.parseAttribute(attr, type);
}

void printTypedAttr(AsmPrinter &printer, Attribute attr, Type type) {
  return parser.printAttributeWithoutType(attr);
}
```

And in TableGen:

```
def FooOp : ... {
  let arguments = (ins AnyAttr:$a);
  let assemblyFormat = [{ custom<TypedAttr>($a, "$_builder.getI1Type()")
                          attr-dict }];
}

def BarOp : ... {
  let arguments = (ins AnyAttr:$a);
  let assemblyFormat = [{ custom<TypedAttr>($a, "$_builder.getIndexType()")
                          attr-dict }];
}
```

Instead of writing two separate sets of custom parsers and printers.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D131603