[FIR2IR] Automatically store IR declaration in its parent upon creation

Previously, creating a declaration with Fir2IrCallableDeclarationsGenerator/
  Fir2IrClassifiersGenerator didn't guarantee that this declaration will
  be actually added to the list of parent class/file declarations, which
  lead to situations when FIR2IR created some declarations in the air
  (mostly fake-overrides)

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

@@ -39,3 +39,4 @@ open enum class Z : Enum<Z> {
     get(): EnumEntries<Z> /* Synthetic body for ENUM_ENTRIES */
 
 }
+