Introduce action to add missing when branches on sealed class

Made via diagnostics NON_EXHAUSTIVE_WHEN_FOR_SEALED_CLASS
with INFO severity and quick-fix
So #KT-17580 Fixed

idea/testData/quickfix/when/addRemainingBranchesSealedStatement.kt

@@ -0,0 +1,16 @@
+// "Add remaining branches" "true"
+// ERROR: Unresolved reference: TODO
+// ERROR: Unresolved reference: TODO
+// ERROR: Unresolved reference: TODO
+sealed class Variant {
+    object Singleton : Variant()
+
+    class Something(val x: Int) : Variant()
+
+    object Another : Variant()
+}
+fun test(v: Variant?) {
+    wh<caret>en(v) {
+        Variant.Singleton -> "s"
+    }
+}

idea/testData/quickfix/when/addRemainingBranchesSealedStatement.kt.after

@@ -0,0 +1,19 @@
+// "Add remaining branches" "true"
+// ERROR: Unresolved reference: TODO
+// ERROR: Unresolved reference: TODO
+// ERROR: Unresolved reference: TODO
+sealed class Variant {
+    object Singleton : Variant()
+
+    class Something(val x: Int) : Variant()
+
+    object Another : Variant()
+}
+fun test(v: Variant?) {
+    when(v) {
+        Variant.Singleton -> "s"
+        is Variant.Something -> TODO()
+        Variant.Another -> TODO()
+        null -> TODO()
+    }
+}