Union of literal types is generalized

The following code should produce the type `'a' | 'b' | 'c'` and not generalize the value to `string`:
```php
use Psl\Type;
use function PHPStan\dumpType;

$literals = Type\union(
    Type\literal_scalar('a'),
    Type\literal_scalar('b'),
    Type\literal_scalar('c'),
);

$validated = $literals->coerce('a');
dumpType($validated); // Dumped type: string
```