Infinite loop in `LookupQualifiedName`

[geoffromer]

Description of the bug:

Infinite loop found by fuzzer.

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

bazel run toolchain/driver:carbon compile test.carbon, where the contents of test.carbon are:

base class B {
}

base class R {
  extend base: B;
}

base class B {
  extend base: B;
  var c: c32;
}

What did you expect to happen?

The compiler terminates in well under a second, probably with some sort of error.

What actually happened?

The compiler runs indefinitely (at least a minute).

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

Initial debugging indicates that this is caused by an infinite loop in LookupQualifiedName, where scope.extended_scopes() consists of the ID of scope itself.