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/ownEquals.kt

@@ -0,0 +1,9 @@
+// PROBLEM: none
+
+sealed class SC {
+    <caret>class U : SC() {
+        override fun equals(other: Any?): Boolean {
+            return this === other
+        }
+    }
+}