[FIR2IR] Configure Unit built-in to have kotlin.Any as a supertype

Without this change, the Unit type does not have Any members declared
in its scope which breaks the expect/actual mapping
(aka we cannot find actual `toString` for expect one).

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/IrBuiltInsOverFir.kt

@@ -95,7 +95,10 @@ class IrBuiltInsOverFir(
     override val nothingType: IrType get() = nothing.type
     override val nothingNType: IrType by lazy { nothingType.makeNullable() }
 
-    private val unit by createClass(kotlinIrPackage, IdSignatureValues.unit, build = { kind = ClassKind.OBJECT; modality = Modality.FINAL })
+    private val unit by createClass(kotlinIrPackage, IdSignatureValues.unit, build = { kind = ClassKind.OBJECT; modality = Modality.FINAL }) {
+        configureSuperTypes()
+        finalizeClassDefinition()
+    }
     override val unitClass: IrClassSymbol get() = unit.klass
     override val unitType: IrType get() = unit.type