Refactor OverrideResolver and OverridingUtil

Fix warnings, simplify code. Untangle a very complex code related to
diagnostics on multiple default values and different names of parameters in
supertypes; fix a bug there, add test

compiler/testData/diagnostics/tests/override/MultipleDefaultsAndNamesInSupertypes.kt

@@ -0,0 +1,13 @@
+trait A {
+    fun foo(x: Int = 42): Int
+}
+
+open class B {
+    fun foo(x: Int = 239) = x
+}
+
+trait C {
+    fun foo(y: Int)
+}
+
+class <!MULTIPLE_DEFAULTS_INHERITED_FROM_SUPERTYPES_WHEN_NO_EXPLICIT_OVERRIDE, DIFFERENT_NAMES_FOR_THE_SAME_PARAMETER_IN_SUPERTYPES!>Z<!> : A, B(), C