Refactor FunctionHandle::isInterfaceDeclaration

- Give it more clear name mayBeUsedAsSuperImplementation
  because defining if it can be used as super-implementation
  this is what it used for
- The meaning is negated, so it's needed to negate its usages and impls
- Also, reuse it in findSuperImplementationForStubDelegation

compiler/backend/src/org/jetbrains/kotlin/codegen/builtinSpecialBridges.kt

@@ -153,19 +153,19 @@ private fun <Signature> findSuperImplementationForStubDelegation(
     areDeclarationAndDefinitionSame: (CallableMemberDescriptor) -> Boolean,
     signatureByDescriptor: (FunctionDescriptor) -> Signature
 ): FunctionDescriptor? {
-    val implementation = findConcreteSuperDeclaration(DescriptorBasedFunctionHandle(function, areDeclarationAndDefinitionSame)).descriptor
+    val implementation = findConcreteSuperDeclaration(DescriptorBasedFunctionHandle(function, areDeclarationAndDefinitionSame))
 
     // Implementation from interface will be generated by common mechanism
-    if (DescriptorUtils.isInterface(implementation.containingDeclaration)) return null
+    if (!implementation.mayBeUsedAsSuperImplementation) return null
 
     // Implementation in super-class already has proper signature
-    if (signatureByDescriptor(function) == signatureByDescriptor(implementation)) return null
+    if (signatureByDescriptor(function) == signatureByDescriptor(implementation.descriptor)) return null
 
     assert(function.modality == Modality.OPEN) {
         "Should generate stubs only for non-abstract built-ins, but ${function.name} is ${function.modality}"
     }
 
-    return implementation
+    return implementation.descriptor
 }
 
 private fun findAllReachableDeclarations(