Implement script instance capturing in script lowering

for regular classes only. Reimplementing the main behavior of the
old BE and implementing few cases on top of it.
#KT-19423 fixed

compiler/ir/ir.tree.impl/src/org/jetbrains/kotlin/ir/declarations/impl/IrClassImpl.kt

@@ -35,7 +35,7 @@ open class IrClassImpl(
     override var visibility: DescriptorVisibility,
     override var modality: Modality,
     override val isCompanion: Boolean = false,
-    override val isInner: Boolean = false,
+    override var isInner: Boolean = false,
     override val isData: Boolean = false,
     override val isExternal: Boolean = false,
     override val isInline: Boolean = false,