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/diagnostics/tests/override/clashesOnInheritance/covariantOverrides.kt

@@ -0,0 +1,37 @@
+interface IBase {
+    fun copy(): IBase
+}
+
+interface ILeft : IBase {
+    override fun copy(): ILeft
+}
+
+open class CLeft : ILeft {
+    override fun copy(): ILeft = CLeft()
+}
+
+interface IRight : IBase {
+    override fun copy(): IRight
+}
+
+interface IDerived : ILeft, IRight {
+    override fun copy(): IDerived
+}
+
+// Error: ILeft::copy and IRight::copy have unrelated return types
+<!RETURN_TYPE_MISMATCH_ON_INHERITANCE, ABSTRACT_MEMBER_NOT_IMPLEMENTED!>class CDerivedInvalid1<!> : ILeft, IRight
+
+// Error: CLeft::copy and IRight::copy have unrelated return types
+<!RETURN_TYPE_MISMATCH_ON_INHERITANCE!>class CDerivedInvalid2<!> : CLeft(), IRight
+
+// OK: CDerived1::copy overrides both ILeft::copy and IRight::copy
+class CDerived1 : ILeft, IRight {
+    override fun copy(): CDerived1 = CDerived1()
+}
+
+// Although ILeft::copy and IRight::copy return types are unrelated, IDerived::copy return type is the most specific of three.
+abstract class CDerived2 : ILeft, IRight, IDerived
+
+class CDerived2a : ILeft, IRight, IDerived {
+    override fun copy(): IDerived = CDerived2a()
+}