Survive NoType when erasing method result types

[odersky]

Fixes #15377

[odersky]

How I fixed it:

I first verified that the second example produced an error and what the error was: A qualifier of a select was Nothing so no member could be found.

I then looked at the crash and instrumented the situation before it happened. It was in TypeErasure:

      eraseResult(tp.resultType) match {
        case rt: MethodType =>
          tp.derivedLambdaType(names ++ rt.paramNames, formals ++ rt.paramInfos, rt.resultType)
        case rt =>
          tp.derivedLambdaType(names, formals, rt)

In fact, rt was NoType since it was a selection with a prefix of type Nothing. I then searched where the type was produced, which was in getStaticForwarderGenericSignature in BCCodeHelpers, which was called in turn by addForwarder.

addForwarder adds static forwarders to modules, and it does not matter if we skip a forwarder. So I tried to
prevent forwarder generation when the erased result type was Nothing. That worked, but what if we have a similar problem in the method argument types? So I thought it was better to prevent forwarder generation if the method type itself did not exist, and for this I needed the additional case in TypeErasure.

        case NoType =>
          // Can happen if we smuggle in a Nothing in the qualifier. Normally we prevent that
          // in Checking.checkMembersOK, but compiler-generated code can bypass this test.
          // See i15377.scala for a test case.
          NoType

It turned out that this check was good enough since addForwarder already avoids generation if the returned type is NoType.