[NI] Support fixation to subtype in completion mode calculator

Allow full compltion mode for return type depending on type parameters in
contravariant position only if they have proper equality constraint.

compiler/testData/diagnostics/tests/inference/completion/equalityConstraintUpstairs.kt

@@ -0,0 +1,17 @@
+// !LANGUAGE: +NewInference
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+class In<in I>(arg: I)
+class Out<out O>(val prop: O)
+class Inv<T>(val prop: T)
+
+interface Upper
+class Lower : Upper
+
+fun <K> id(arg: K): K = arg
+
+fun test(lower: Lower) {
+    id<Inv<Upper>>(Inv(lower))
+    id<In<Upper>>(In(lower))
+    id<Out<Upper>>(Out(lower))
+}