KT-4198 On covariant projection, do not throw whole var's away

#KT-4198 Fixed

compiler/testData/diagnostics/tests/generics/varProjection/setterProjectedOutPlusAssignDefined.kt

@@ -0,0 +1,12 @@
+trait Tr<T> {
+    var v: T
+}
+
+class C {
+    fun plusAssign(<!UNUSED_PARAMETER!>c<!>: C) {}
+}
+
+fun test(t: Tr<out C>) {
+    // No error because no real assignment happens
+    t.v += null!!
+}