Refactor DelegationChecker and related code

- move getDelegates from CodegenUtil to DelegationResolver, reuse it in
  DelegationChecker
- use getAllOverriddenDescriptors + filterOutOverridden instead of a manual DFS

compiler/backend-common/src/org/jetbrains/kotlin/backend/common/CodegenUtil.kt

@@ -24,51 +24,10 @@ import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.psi.*
 import org.jetbrains.kotlin.resolve.BindingContext
 import org.jetbrains.kotlin.resolve.DescriptorUtils
-import org.jetbrains.kotlin.resolve.MemberComparator
-import org.jetbrains.kotlin.resolve.OverridingUtil
 import org.jetbrains.kotlin.resolve.calls.callUtil.getResolvedCall
 import org.jetbrains.kotlin.types.KotlinType
-import org.jetbrains.kotlin.types.isDynamic
-import org.jetbrains.kotlin.utils.keysToMapExceptNulls
 
 object CodegenUtil {
-    // class Foo : Bar by baz
-    //   descriptor = Foo
-    //   toInterface = Bar
-    //   delegateExpressionType = typeof(baz)
-    // return Map<member of Foo, corresponding member of typeOf(baz)>
-    @JvmStatic
-    fun getDelegates(
-            descriptor: ClassDescriptor,
-            toInterface: ClassDescriptor,
-            delegateExpressionType: KotlinType? = null
-    ): Map<CallableMemberDescriptor, CallableDescriptor> {
-        if (delegateExpressionType?.isDynamic() ?: false) return emptyMap()
-
-        return descriptor.defaultType.memberScope.getContributedDescriptors().asSequence()
-                .filterIsInstance<CallableMemberDescriptor>()
-                .filter { it.kind == CallableMemberDescriptor.Kind.DELEGATION }
-                .asIterable()
-                .sortedWith(MemberComparator.INSTANCE)
-                .keysToMapExceptNulls { delegatingMember ->
-                    val actualDelegates = DescriptorUtils.getAllOverriddenDescriptors(delegatingMember)
-                            .filter { it.containingDeclaration == toInterface }
-                            .map { overriddenDescriptor ->
-                                val scope = (delegateExpressionType ?: toInterface.defaultType).memberScope
-                                val name = overriddenDescriptor.name
-
-                                // this is the actual member of delegateExpressionType that we are delegating to
-                                (scope.getContributedFunctions(name, NoLookupLocation.FROM_BACKEND) +
-                                 scope.getContributedVariables(name, NoLookupLocation.FROM_BACKEND))
-                                        .firstOrNull { it == overriddenDescriptor || OverridingUtil.overrides(it, overriddenDescriptor) }
-                             }
-
-                    assert(actualDelegates.size <= 1) { "Many delegates found for $delegatingMember: $actualDelegates" }
-
-                    actualDelegates.firstOrNull()
-                }
-    }
-
     @JvmStatic
     fun getDelegatePropertyIfAny(
             expression: KtExpression, classDescriptor: ClassDescriptor, bindingContext: BindingContext