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/multiFileLocalInspections/convertSealedSubClassToObject/convertCallableReferenceUsages/convertCallableReferenceUsages.test

@@ -1,5 +1,5 @@
 {
   "mainFile": "Seal.kt",
   "inspectionClass": "org.jetbrains.kotlin.idea.inspections.CanSealedSubClassBeObjectInspection",
-  "fix": "Convert Sealed Sub-class to Object"
+  "fix": "Convert sealed sub-class to object"
 }

idea/testData/multiFileLocalInspections/convertSealedSubClassToObject/convertInOtherFiles/convertInOtherFiles.test

@@ -1,5 +1,5 @@
 {
   "mainFile": "Seal.kt",
   "inspectionClass": "org.jetbrains.kotlin.idea.inspections.CanSealedSubClassBeObjectInspection",
-  "fix": "Convert Sealed Sub-class to Object"
+  "fix": "Convert sealed sub-class to object"
 }