Sealed sub-class -> object: handle equals, suggest "add equals"

Before this commit, sealed sub-class without state was considered
a style issue.
After this commit, sealed sub-class without state AND custom equals
is considered a probable bug,
because comparison of its instances is very fragile.
Alternative fix (generate equals & hashCode by identity) is added.

idea/testData/inspectionsLocal/convertSealedSubClassToObject/noEquals.kt.after

@@ -0,0 +1,21 @@
+// FIX: Generate equals & hashCode by identity
+
+abstract class Base {
+    open val prop: Int
+        get() = 13
+}
+
+sealed class SC : Base() {
+    class U : SC() {
+        override fun equals(other: Any?): Boolean {
+            return this === other
+        }
+
+        override fun hashCode(): Int {
+            return System.identityHashCode(this)
+        }
+    }
+
+    override val prop: Int
+        get() = 42
+}