Report resolution ambiguity on '+=' by taking into account full resolution result including post type checking for '+'

^KT-45503 Fixed

compiler/testData/diagnostics/tests/operatorsOverloading/kt45503_1.kt

@@ -0,0 +1,15 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+// WITH_RUNTIME
+// SKIP_TXT
+
+class A<T>
+class B<T>(val x: MutableList<T>) : MutableList<T> by x
+
+class C {
+    operator fun <T> get(k: A<T>): T = TODO()
+    operator fun <T> set(k: A<T>, v: T): Unit = TODO()
+}
+
+fun foo() {
+    C()[A<B<Int>>()] += 2
+}