Allow Init Checker to look within anonymous classes

[bishabosha]

class Wrap {
  class E
  object E {
    final val A = new E {}
    val $values = Array(A)
  }
}

produces warning with -Ycheck-init

-- Warning: initcheck.scala:5:23 ----------------------------------
5 |    val $values = Array(A)
  |                        ^
  |Promoting the value under initialization to fully-initialized. Calling trace:
  | -> val $values = Array(A)	[ initcheck.scala:5 ]
1 warning found

when it could look inside to prove it is safe.

One example that is not safe and will lead to NPE in initialisation of object E:

class Wrap {                                                                                                     
  def qux[T](e: E[T]) = e.foo

  abstract class E[+T] { def foo: T }
  object E {
    final val A: E[Nothing] = new E { def foo = ref }
    val ref = qux(A)
  }
}

as pointed out in #9665 (comment)

if ref is made lazy then there is no warning on Array(A) and object E can be constructed correctly.

[propensive]

This issue is referenced in a comment here: https://github.com/lampepfl/dotty/blob/4a6ec60f13be412e11bb81b3c3028cc47dc1c2ec/compiler/src/dotty/tools/dotc/ast/DesugarEnums.scala#L122-L127

Given that this issue is closed, it might be possible to address the comment now.

[liufengyun]

This issue will be fixed by #11533. @natsukagami Could you add it as a test:

• tests/init/pos/i10549a.scala
• tests/init/neg/i10549b.scala