Rename ClassKind.isObject -> isSingleton

"isObject" gives a false impression that this is a check if the kind is OBJECT
Minor, NamedMembers.toString()
Delete ExpressionPosition

It had been deprecated in 838c408
Use StorageManager in AbstractClassDescriptor to create a lazy value
Pull up getClassObjectType()
Pull up getUnsubstitutedInnerClassesScope()
Cleanup ClassDescriptorImpl & ErrorClassDescriptor

ClassDescriptorImpl now extends ClassDescriptorBase and reuses most of its
functionality. ErrorClassDescriptor now is initialized in its own constructor.
Inline trivial arguments everywhere
Pull up getThisAsReceiverParameter()