implement type inference for union-types in sql queries

[staabm]

refs #276 (comment)

• PDO implement type inference for union-types in sql queries #290
• Mysqli mysqli: implement type inference for union-types in sql queries #294
• Doctrine DBAL doctrine-dbal: implement union query types #295

example case

class Foo
{
    public function doFoo(PDO $pdo)
    {
        $queries = ['SELECT adaid FROM ada', 'SELECT email FROM ada'];

        foreach ($queries as $query) {
            $stmt = $pdo->prepare($query);
            $result = $stmt->fetch(PDO::FETCH_ASSOC);
            assertType('array{adaid: int<0, 4294967295>}|array{email: string}|false', $result);
        }
    }
}