Fix SimplXmlElement and GMP casts

[VincentLanglet]

Closes phpstan/phpstan#14136

I tried https://3v4l.org/BtQlq#v8.4.18

Fun fact, I discovered that GMP is not castable to bool for PHP < 8.4
See https://wiki.php.net/rfc/fix_up_bcmath_number_class

But:

• Currently ObjectType::toBool was returning ConstantBoolean(true), so returning a BooleanType is still better (cause for PHP >= 8.4 it can be false !)
• boolval is defined as accepting mixed in the resourceMap
• There is currently no way to do a condition about the PHPVersion inside ObjectType::toBool
• Nobody reported the issue yet

So I think it's a good enough improvement yet.

[phpstan-bot]

This pull request has been marked as ready for review.