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/delegatedProperty/delegatedPropertyOverridedInTraitTypeMismatch.kt

@@ -7,7 +7,7 @@ interface A {
 }
 
 class AImpl: A  {
-    override val <!RETURN_TYPE_MISMATCH_ON_OVERRIDE!>prop<!> by Delegate()
+    override val <!PROPERTY_TYPE_MISMATCH_ON_OVERRIDE!>prop<!> by Delegate()
 }
 
 fun foo() {