[mlir][ods] Fix verification of attribute + colon type ambiguity

An attribute without a type builder followed by a colon in an assembly format is potentially ambiguous because the parser will read ahead to parse the colon-type and pass this as the type argument to the attribute's constructor.

However, the previous verifier that checks for this ambiguity erroneously produces an error in the case of

```
let assemblyFormat = "( `(` $attr `)` )? `:`";
```

This patch fixes the bug by implementing a checker that correctly handles all edge cases, including very strange assembly formats like:

```
let assemblyFormat = "( `(` $attr ) : (`>`)? attr-dict (`>` $a^) : (`<`)? `:`";
```

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D125445