FIR2IR: Fix IAE for case of local override of a method with defaults

It was happening because for MyClass.foo we didn't set overriddenSymbols
properly because in ClassMemberGenerator.convertFunctionContent we
used incorrect containingFirClass that was pointing to anonymous class
instead of MyClass.

^KT-58902 Fixed

compiler/testData/codegen/box/fir/localOverrideWithDefaultInLocalOverridden.kt

@@ -0,0 +1,15 @@
+// ISSUE: KT-58902
+
+fun box(): String {
+    return run {
+        open class A {
+            open fun foo(x: String, y: String? = null): String = x + (y ?: "K")
+        }
+
+        class MyClass : A() {
+            override fun foo(x: String, y: String?) = super.foo(x, y)
+        }
+
+        MyClass()
+    }.foo("O")
+}