added variance handling to constraint system

compiler/testData/diagnostics/tests/inference/dependantOnVarianceNullable.kt

@@ -0,0 +1,14 @@
+package b
+//+JDK
+
+import java.util.*
+import java.util.Collections.*
+
+fun foo(list: List<String>) : String {
+    val w : String = max(list, comparator<String?> {o1, o2 -> 1
+    })!!
+    return w
+}
+
+//from library
+fun <T> comparator(<!UNUSED_PARAMETER!>fn<!>: (T,T) -> Int): Comparator<T> {<!NO_RETURN_IN_FUNCTION_WITH_BLOCK_BODY!>}<!>