Change "most specific return type" definition for fake overrides.

Given overridden descriptors D = d[i].
1. Find D*, subset of D:
  returnType(d* from D*) <: returnType(d) for each d from D.
  Always prefer var to val.
2. Prefer non-flexible return type to flexible.

Check for var/val overrides properly
(NB: this will report PROPERTY_TYPE_MISMATCH_ON_OVERRIDE
for all properties, not just overrides involving vars as it was before).

compiler/testData/diagnostics/tests/override/clashesOnInheritance/genericWithUpperBound.kt

@@ -0,0 +1,56 @@
+// FILE: JFooWithUpperBound.java
+public interface JFooWithUpperBound<T extends IBase> {
+    T foo();
+}
+
+// FILE: JFooWithUpperBoundDerived.java
+public interface JFooWithUpperBoundDerived<T extends IBase> extends JFooWithUpperBound<T> {
+}
+
+// FILE: JCFooWithUpperBound.java
+public class JCFooWithUpperBound<T extends IBase> {
+    public T foo() {
+        return null;
+    }
+}
+
+// FILE: JCFooWithUpperBoundDerived.java
+public class JCFooWithUpperBoundDerived<T extends IBase> extends JCFooWithUpperBound<T> {
+}
+
+// FILE: K.kt
+interface IBase
+
+interface IDerived : IBase
+
+interface IFooWithUpperBound<T : IBase> {
+    fun foo(): T
+}
+
+interface IFooT<T> {
+    fun foo(): T
+}
+
+interface IFoo {
+    fun foo(): IBase
+}
+
+interface IFooDerived : IFoo {
+    override fun foo(): IDerived
+}
+
+interface IFooWithUpperBoundDerived<T : IBase> : IFooWithUpperBound<T>
+
+interface Test1<T : IBase> : IFooWithUpperBound<T>, IFoo
+
+interface Test2<T : IBase> : IFooT<T>, IFoo
+
+interface Test3<T : IDerived> : IFooWithUpperBoundDerived<T>, IFooDerived
+
+interface Test4<T : IBase> : JFooWithUpperBound<T>, IFoo
+
+interface Test5<T : IDerived> : JFooWithUpperBoundDerived<T>, IFooDerived
+
+class Test6<T : IBase> : JCFooWithUpperBound<T>(), IFoo
+
+class Test7<T : IDerived> : JCFooWithUpperBoundDerived<T>(), IFooDerived