Do not allow compareTo() to return Int?

compiler/testData/diagnostics/tests/operatorsOverloading/compareToNullable.kt

@@ -0,0 +1,12 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+class C {
+    fun compareTo(c: C): Int? = null
+}
+
+fun test(c: C) {
+    c <!COMPARE_TO_TYPE_MISMATCH!><<!> c
+    c <!COMPARE_TO_TYPE_MISMATCH!><=<!> c
+    c <!COMPARE_TO_TYPE_MISMATCH!>>=<!> c
+    c <!COMPARE_TO_TYPE_MISMATCH!>><!> c
+}

compiler/testData/diagnostics/tests/operatorsOverloading/compareToNullable.txt

@@ -0,0 +1,11 @@
+package
+
+internal fun test(/*0*/ c: C): kotlin.Unit
+
+internal final class C {
+    public constructor C()
+    internal final fun compareTo(/*0*/ c: C): kotlin.Int?
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}