Avoid early-binding on unresolved types

[nikic]

Fixes bug #76451, and more importantly lays necessary groundwork for covariant/contravariant types. Bug #76451 is just an edge case, but once covariance is introduced this will become a common problem instead.

The core problem is that the inheritance code currently simply uses zend_lookup_class when checking whether two types (that are not string equal) are the same due to aliasing (or in the future: compatible through variance) and treats non-existing classes as an error. This has two problems:

• If the class is early bound and one of the types is not found, this is treated as an error. Instead no early binding should be performed, as the types may be defined at runtime.
• Again during early binding, types outside the current file will be used, thus introducing an illegal state dependence in opcache.

This PR solves the problem by

• Introducing an UNRESOLVED state when performing a method compatibility check. This indicates that necessary types couldn't be loaded. At runtime this is the same as an error, while at compile-time it means that early binding cannot be performed.
• Introducing the necessary checks to limit class loading to the local file at compile time.
• Before attempting to perform early-binding with a parent class, do a pass over the methods to check that there are no UNRESOLVED incompatibilities. This is rather ugly, but I couldn't think of a better way to do it. It would be great if we could just try to run inheritance and abort if we encounter UNRESOLVED, but as inheritance is destructive, we need to detect this condition before we run it.

[nikic]

@dstogov Please have a look at this. Once this issue is resolved, the actual covariance implementation will be simple.

[dstogov]

This patch affects preloading. Fixing it may disclose other problems.
make test TESTS="--preload Zend/tests/bug76451.phpt"

More optimal solution (without double-checks) would be great.

I'm not sure, if covariant/contravariant types may work without run-time checks, and how this patch would help them.

[dstogov]

Do we need this lookup. if "ce" is NULL? We didn't do it before.

[dstogov]

In case we need this only for methods with "class" type hints, we may set special fn_flag to avoid duplicate check for others. We may also reuse the same flag for classes to completely eliminate this loop. (At least we may reuse ZEND_ACC_HAS_TYPE_HINTS & ZEND_ACC_HAS_RETURN_TYPE for methods).

[dstogov]

On the other hand, if we check everything in zend_can_early_bind() and everything is fine (INHERITANCE_SUCCESS), we may avoid the same checks in do_bind_class(). If zend_do_perform_implementation_check() returns INHERITANCE_ERROR, we may throw error immediately.

[nikic]

Part of #4185 now.