Fix #75765 Exception on extend of undefined class

[timurib]

Extending an undefined class causes the fatal error instead of throwing Error exception.
Relevant issue: https://bugs.php.net/bug.php?id=75765

The previous patch contained a fix for "implements" handling, but it had errors, so I removed all related with interfaces and made a new PR.

[nikic]

Merged as a484b9a into master. Thanks!

To support the same for interfaces/traits, the basic idea would be to have a separate opcode like COMMIT_CLASS, which separates the addition of the class to the hashtable from the binding procedure, in which case all binding errors could become exceptions. This part I would assume to be relatively straightforward. The issue is that the inheritance logic also modifies the class in-place, this means that it's not possible to run inheritance on a class entry twice (e.g. after defining a necessary interface that previously threw). Solving that part wouldn't be so simple.