Fuzzer issue: extending a member class as a base class

[josh11b]

Description of the bug:

Proposal #2760 moved the declaration of a base class from the class declaration line into the body of the class definition. This change was implemented in Explorer in #2906 . However, this change did not account for the consequence of this change that the base class expression could now refer to something declared earlier in class scope. The implementation in #2906 of TypeChecker::DeclareClassDeclaration(...) in explorer/interpreter/type_checker.cpp type checks the base class expression before DeclareDeclaration(...) can be called on the members. If the base class expression contains a reference to a member (such as a member class), it fails when it tries to access that member's static_type, which hasn't been set yet.

Unfortunately, this code is difficult to reorder, since the members can refer to Self which is a NominalClassType created in this function, with base_class as a parameter, which we don't know until we've processed the extend base declaration.

What did you do, or what's a simple way to reproduce the bug?

This triggers the bug:

package P impl;

class C {
  class D { }
  extend base: D;
}

fn Main() -> i32 { return 0; }

This does not:

package P impl;

class D { }
class C {
  extend base: D;
}

fn Main() -> i32 { return 0; }

What did you expect to happen?

No crash. This code should instead produce some compilation error about trying to extend a class that is final.

What actually happened?

Crash:

... optional:943: _LIBCPP_ASSERT 'this->has_value()' failed. optional operator* called on a disengaged value

Any other information, logs, or outputs that you want to share?

No response