Narrow ReflectionEnum::getBackingType after ReflectionEnum::isBacked

[RobertMe]

Fixes phpstan/phpstan#10167

[phpstan-bot]

You've opened the pull request against the latest branch 1.11.x. If your code is relevant on 1.10.x and you want it to be released sooner, please rebase your pull request and change its target to 1.10.x.

[RobertMe]

I'm guessing the tests could be "improved" (not sure whether it should be at root of the file, if it actually can be added in this same file).
Furthermore I "discovered" that the return type actually is dependent on the PHP version (PHP 8.2+ returns ReflectionNamedType|null while lower versions return ReflectionType|null) so that might require some fix/improvement too.

[staabm]

Why did you close the PR?

[RobertMe]

Why did you close the PR?

Because it's not finished (failing tests); I was unsure how to fix the 8.2+ vs 8.1 differences, also because of a lack of input (/"support"); I didn't even continue using the code which lead to this "issue" (so in the end I personally didn't even need it anymore, although I normally would finish PRs that I started, even when I wouldn't need them anymore);
And, to be honest, foremost because of how my more recent issue and accompanying PR were handled and I'm not even sure whether I like to continue using PHPStan, or whether to switch to Psalm (although I might regret that if Psalm doesn't validate and infer types from SQL queries 😉).

[staabm]

I see, thx. I will have a look at the issue