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-common/src/org/jetbrains/kotlin/backend/common/bridges/bridges.kt

@@ -27,7 +27,7 @@ interface FunctionHandle {
     * class ones (see [findConcreteSuperDeclaration] method in bridges.kt).
     * Note that interface methods with body compiled to jvm 8 target are assumed to be non-abstract in bridges method calculation
     * (more details in [DescriptorBasedFunctionHandle.isBodyOwner] comment).*/
-    val isInterfaceDeclaration: Boolean
+    val mayBeUsedAsSuperImplementation: Boolean
 
     fun getOverridden(): Iterable<FunctionHandle>
 }
@@ -114,7 +114,7 @@ fun <Function : FunctionHandle> findConcreteSuperDeclaration(function: Function)
     }
     result.removeAll(toRemove)
 
-    val concreteRelevantDeclarations = result.filter { !it.isAbstract && !it.isInterfaceDeclaration }
+    val concreteRelevantDeclarations = result.filter { !it.isAbstract && it.mayBeUsedAsSuperImplementation }
     if (concreteRelevantDeclarations.size != 1) {
         error("Concrete fake override $function should have exactly one concrete super-declaration: $concreteRelevantDeclarations")
     }

compiler/backend-common/src/org/jetbrains/kotlin/backend/common/bridges/impl.kt

@@ -72,8 +72,8 @@ class DescriptorBasedFunctionHandle(
     override val isAbstract: Boolean =
         descriptor.modality == Modality.ABSTRACT || !areDeclarationAndDefinitionSame(descriptor)
 
-    override val isInterfaceDeclaration: Boolean
-        get() = DescriptorUtils.isInterface(descriptor.containingDeclaration)
+    override val mayBeUsedAsSuperImplementation: Boolean =
+        !DescriptorUtils.isInterface(descriptor.containingDeclaration)
 
     override fun getOverridden() = overridden