[IR] Make descriptor parameter optional for IrFileSymbolImpl and IrExternalPackageFragmentSymbolImpl

Remove DescriptorlessIrFileSymbol and use IrFileSymbolImpl instead.

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/symbols/impl/IrPrivateSymbolBase.kt

@@ -1,17 +1,6 @@
 /*
- * Copyright 2010-2017 JetBrains s.r.o.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
 package org.jetbrains.kotlin.ir.symbols.impl
@@ -83,11 +72,11 @@ abstract class IrBindableSymbolBase<out D : DeclarationDescriptor, B : IrSymbolO
         get() = _owner != null
 }
 
-class IrFileSymbolImpl(descriptor: PackageFragmentDescriptor) :
+class IrFileSymbolImpl(descriptor: PackageFragmentDescriptor? = null) :
     IrBindableSymbolBase<PackageFragmentDescriptor, IrFile>(descriptor),
     IrFileSymbol
 
-class IrExternalPackageFragmentSymbolImpl(descriptor: PackageFragmentDescriptor) :
+class IrExternalPackageFragmentSymbolImpl(descriptor: PackageFragmentDescriptor? = null) :
     IrBindableSymbolBase<PackageFragmentDescriptor, IrExternalPackageFragment>(descriptor),
     IrExternalPackageFragmentSymbol