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/baseEquals.kt

@@ -0,0 +1,18 @@
+// PROBLEM: none
+
+abstract class Base {
+    open val prop: Int
+        get() = 13
+
+    override fun equals(other: Any?): Boolean {
+        if (other !is Base) return false
+        return prop == other.prop
+    }
+}
+
+sealed class SC : Base() {
+    <caret>class U : SC()
+
+    override val prop: Int
+        get() = 42
+}