**Use case**
`SELECT POSITION('/' IN s) FROM (SELECT 'Hello/World' AS s);`

The same as `position(s, '/')`

{
    Field res;
    if (!pos.isValid())
        return false;
    UInt64 x = 0;
    ReadBufferFromMemory in(pos->begin, pos->size());
    if (!tryReadIntText(x, in) || in.count() != pos->size())
    {
        expected.add(pos, "unsigned integer");
        return false;
    }
    res = x;
    auto literal = std::make_shared<ASTLiteral>(res);
    literal->begin = pos;
    literal->end = ++pos;
    node = literal;
    return true;
}
bool ParserStringLiteral::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)