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/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestGenerated.java

@@ -11360,6 +11360,12 @@ public class DiagnosticsTestGenerated extends AbstractDiagnosticsTest {
                     doTest(fileName);
                 }
 
+                @TestMetadata("genericWithUpperBound.kt")
+                public void testGenericWithUpperBound() throws Exception {
+                    String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/override/clashesOnInheritance/genericWithUpperBound.kt");
+                    doTest(fileName);
+                }
+
                 @TestMetadata("returnTypeMismatch.kt")
                 public void testReturnTypeMismatch() throws Exception {
                     String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/tests/override/clashesOnInheritance/returnTypeMismatch.kt");