KT-1934 Check inherited signatures for compatibility

Overridden signatures should have compatible return types
(equal types for 'var').
Only relevant overrides should be taken into account.
Refactor inherited signatures check,
introduce a strategy interface for problem reporting.

compiler/testData/codegen/box/bridges/kt1939.kt

@@ -3,7 +3,7 @@ abstract class Foo<T> {
 }
 
 interface Tr {
-    fun hello(s : String)
+    fun hello(s : String): String
 }
 
 class Bar: Foo<String>(), Tr {