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

#KT-4198 Fixed

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

@@ -0,0 +1,7 @@
+trait Tr<T> {
+    var v: T
+}
+
+fun test(t: Tr<out String>) {
+    <!SETTER_PROJECTED_OUT!>t.v<!> += null!!
+}