KT-9550: distinguish signature conflict on override from signature conflict of inherited members.

Add a common OverridingStrategy class for cases where we don't care (yet) about precise diagnostics.

compiler/testData/diagnostics/tests/override/clashesOnInheritance/kt9550.kt

@@ -0,0 +1,13 @@
+interface A {
+    fun <T> foo()
+    fun <T> bar()
+}
+
+interface B {
+    fun foo()
+    fun bar()
+}
+
+<!CONFLICTING_INHERITED_MEMBERS!>interface C1<!> : A, B {
+    <!CONFLICTING_OVERLOADS!>override fun bar()<!>
+}

compiler/testData/diagnostics/tests/override/clashesOnInheritance/kt9550.txt

@@ -0,0 +1,25 @@
+package
+
+public interface A {
+    public abstract fun </*0*/ T> bar(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract fun </*0*/ T> foo(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface B {
+    public abstract fun bar(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract fun foo(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}
+
+public interface C1 : A, B {
+    public abstract override /*1*/ fun bar(): kotlin.Unit
+    public open override /*2*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract override /*1*/ /*fake_override*/ fun </*0*/ T> foo(): kotlin.Unit
+    public open override /*2*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*2*/ /*fake_override*/ fun toString(): kotlin.String
+}