[flang] [OpenMP] adjust for PR#753 `OmpObject` parse tree change

PR#753 changed the `OmpObject` to be:

```
struct OmpObject {
  UNION_CLASS_BOILERPLATE(OmpObject);
  std::variant<Designator, /*common block*/ Name> u;
};
```

This commit adjust the logic flow for `ResolveOmpObject` based on
the above parse tree change.

For an `OmpObject`,
```
if (Designator) {
  if (DataRef.Name) {
    // Resolve Names
  } else {
    // ResolveDesignator
    // AnalyzeExpr
    if (Designator.Substring) {
      // Error
    }
    // other checks like StructureComponent, ArrayElement, etc.
  }
} else if (Name) {
  // Resolve COMMON block
}
```

Original-commit: flang-compiler/f18@e5c1b92c15e5e8aee202d3469e337893510cfe21