Java 8 rules for method overrides:

- base class method wins against a (default) interface method,
so an abstract base class method should always be implemented
in a derived class;

- interface methods clash regardless of abstract/default
with possibly undefined behavior at run-time,
so a class or interface should always define its own method
for methods inherited from multiple interfaces and not from base class;

- meaningful diagnostics for class inheriting conflicting JVM signatures.
Since no override will happen under Java 8 rules,
ACCIDENTAL_OVERRIDE is misleading for this case;

- update testData.

compiler/testData/diagnostics/tests/java8Overrides/abstractBaseClassMemberNotImplemented.kt

@@ -0,0 +1,9 @@
+abstract class ALeft {
+    abstract fun foo()
+}
+
+interface IRight {
+    fun foo() {}
+}
+
+<!ABSTRACT_CLASS_MEMBER_NOT_IMPLEMENTED!>class CDerived<!> : ALeft(), IRight