FIR: Report UNSAFE_OPERATOR_CALL for augmented assignments (was

reporting UNSAFE_CALL).

compiler/testData/diagnostics/tests/nullabilityAndSmartCasts/augmentedAssignment.kt

@@ -0,0 +1,17 @@
+class A {
+    operator fun plusAssign(s: String) {}
+}
+
+fun test() {
+    var a: A? = A()
+    a <!UNSAFE_OPERATOR_CALL!>+=<!> ""
+}
+
+class B {
+    operator fun plus(other: B) = this
+}
+
+fun test2() {
+    var b: B? = B()
+    b <!UNSAFE_OPERATOR_CALL!>+=<!> B()
+}