[Test] Operator ambiguity check with 'set'

When using `x[y] += z` the spec mandates attempting all possible combinations.

^KT-62138 Fixed

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

@@ -0,0 +1,22 @@
+// FIR_IDENTICAL
+// ISSUE: KT-62138
+class HashMap<K, V>(
+    private val defaultValue: V
+) {
+    operator fun get(key: K): V = defaultValue
+    operator fun set(key: K, value: V) { }
+}
+
+private class X
+
+private operator fun X?.plus(p: Int) = X()
+private operator fun X?.plusAssign(p: Int) { }
+
+class C {
+    private val map = HashMap<String, X>(defaultValue = X())
+
+    fun f(): Any? {
+        map[""] <!ASSIGN_OPERATOR_AMBIGUITY!>+=<!> 1
+        return map[""]
+    }
+}