FIR2IR: handle 'this' as reference to outer object

compiler/testData/ir/irText/singletons/enumEntry.fir.txt

@@ -29,7 +29,7 @@ FILE fqName:<root> fileName:/enumEntry.kt
             $this: VALUE_PARAMETER name:<this> type:<root>.Z.ENTRY.A
             BLOCK_BODY
               CALL 'public final fun test (): kotlin.Unit declared in <root>.Z.ENTRY' type=kotlin.Unit origin=null
-                $this: GET_OBJECT 'CLASS ENUM_ENTRY name:ENTRY modality:FINAL visibility:private superTypes:[<root>.Z]' type=<root>.Z.ENTRY
+                $this: GET_VAR '<this>: <root>.Z.ENTRY declared in <root>.Z.ENTRY' type=<root>.Z.ENTRY origin=null
           FUN FAKE_OVERRIDE name:equals visibility:public modality:OPEN <> ($this:kotlin.Any, other:kotlin.Any?) returnType:kotlin.Boolean [fake_override,operator]
             overridden:
               public open fun equals (other: kotlin.Any?): kotlin.Boolean [operator] declared in kotlin.Any