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/convertSubClassWithModifiers.kt

@@ -1,3 +1,5 @@
+// FIX: Convert sealed sub-class to object
+
 sealed class Sealed
 
 private <caret>class SubSealed : Sealed()