Fix string types sorting

[VincentLanglet]

Needed for #3438 (review)
or testUnion and testUnionReversed doesn't give the same result.

As seen in
https://phpstan.org/r/f9b06e12-01e6-45c2-92dc-2fcd514a5dbb
PHPStan is not consistent about union of strings.

I'm not familiar with the UnionTypeHelper method, so I'm not sure if

if ($a->isString()->yes() && $b->isString()->yes()) {	return self::compareStrings($a->describe(VerbosityLevel::value()), $b->describe(VerbosityLevel::value()));	} 

is the best fix or if something more generic like

instanceof IntersectionType 

should be preferred.

[ondrejmirtes]

Thank you!