K2: Fix false-positive OVERLOAD_RESOLUTION_AMBIGUITY

It's been introduced in the previous commit
("K2: Simplify handling mixed smartcast vs. original candidates")

Because previously, it was assumed wrongly that each next level of
ConeCallConflictResolver filter out the candidates that are 100% less
applicable/specific, but the main one (ConeOverloadConflictResolver)
either leaves the single candidate or the whole same set, thus at
FilteringOutOriginalInPresenceOfSmartCastConeCallConflictResolver
we've got 4 candidates only two of which we might filter out.

compiler/testData/diagnostics/tests/overload/overloadsFromCurrentAndSuperClass.kt

@@ -0,0 +1,23 @@
+// SKIP_TXT
+// FIR_IDENTICAL
+
+abstract class A {
+    open public fun foo(x: Any) {}
+    open public fun foo(x: String) {}
+}
+
+class B : A() {
+    override fun foo(x: Any) {
+        super.foo(x)
+    }
+
+    override fun foo(x: String) {
+        super.foo(x)
+    }
+}
+
+fun bar(a: A) {
+    if (a is B) {
+        a.foo("")
+    }
+}

compiler/testData/diagnostics/tests/overload/overloadsFromCurrentAndSuperClassWithReturnType.kt

@@ -0,0 +1,18 @@
+// SKIP_TXT
+// FIR_IDENTICAL
+
+abstract class A {
+    open public fun foo(x: Any): Any = x
+    open public fun foo(x: String): String = x
+}
+
+class B : A() {
+    override fun foo(x: Any): Any = x
+    override fun foo(x: String): String = x
+}
+
+fun bar(a: A) {
+    if (a is B) {
+        a.foo("").length
+    }
+}