fix a case of fake override of abstract and non-abstract function

===
open class Ccc() {
    fun foo() = 1
}

trait Ttt {
    fun foo(): Int
}

class Zzz() : Ccc(), Ttt // there must not be an error here
===

Reported by Andrey Breslav as a bug of Comparator implementation:

===
fun comparator<T>(f: (T, T) -> Int): Comparator<T> = object : Comparator<T>, Object() {
    override fun compare(o1: T, o2: T): Int = f(o1, o2)
}
===

compiler/frontend/src/org/jetbrains/jet/lang/resolve/OverrideResolver.java

@@ -357,7 +357,15 @@ public class OverrideResolver {
                     abstractNoImpl.add(single);
                 }
             } else {
-                manyImpl.addAll(overridenDeclarations);
+                List<CallableMemberDescriptor> nonAbstractManyImpl = Lists.newArrayList();
+                for (CallableMemberDescriptor overriden : overridenDeclarations) {
+                    if (overriden.getModality() != Modality.ABSTRACT) {
+                        nonAbstractManyImpl.add(overriden);
+                    }
+                }
+                if (nonAbstractManyImpl.size() > 1) {
+                    manyImpl.addAll(nonAbstractManyImpl);
+                }
             }
         }
     }

compiler/testData/diagnostics/tests/override/FakeOverrideAbstractAndNonAbstractFun.jet

@@ -0,0 +1,9 @@
+open class Ccc() {
+    fun foo() = 1
+}
+
+trait Ttt {
+    fun foo(): Int
+}
+
+class Zzz() : Ccc(), Ttt