Detect more possibly-impossible offset

[VincentLanglet]

Closes phpstan/phpstan#1061

[ondrejmirtes]

I feel like the error could always be reported on level 7+.

The difference is that the reportPossiblyUndefined... option is for offsets like string, but in this case this is about a union and one of the union parts never exists on the constant array so it'd be a good fit for L7+ without any extra option.

[VincentLanglet]

I feel like I'm near the solution but I have a question for you first @ondrejmirtes.

The integration test show the current situation, we have now duplicates errors

Offset int|object might not exist on array|string.
Possibly invalid array key type int|object.

I know how to fix it, but I just want to show you it's also the current behavior when reportPossiblyNonexistentGeneralArrayOffset or reportPossiblyNonexistentConstantArrayOffset is enabled:
https://phpstan.org/r/b61c0c45-acbf-4493-90d4-18bfdedb1bb8

Do you want me to fix it for all three cases ?