[FIR2IR] Properly calculate dispatch receiver type for fake overrides

If some function is not fake-override, then its type should be just
  default type of containing class
For fake overrides the default type calculated in the following way:
1. Find first overridden function, which is not fake override
2. Take its containing class
3. Find supertype of current containing class with type constructor of
   class from step 2

^KT-60252 Fixed

compiler/testData/ir/irText/expressions/thisOfGenericOuterClass.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 class Outer<T>(val x: T) {
     open inner class Inner(val y: Int)
 }