Change MISSING_CONSTRUCTOR_KEYWORD severity to error

And adjust testData

compiler/testData/codegen/box/defaultArguments/constructor/kt3060.kt

@@ -1,4 +1,4 @@
-class Foo private(val param: String = "OK") {
+class Foo private constructor(val param: String = "OK") {
     companion object {
         val s = Foo()
     }

compiler/testData/codegen/box/defaultArguments/private/primaryConstructor.kt

@@ -1,6 +1,6 @@
 var state: String = "Fail"
 
-class A private(x: String = "OK") {
+class A private constructor(x: String = "OK") {
     init {
         state = x
     }

compiler/testData/codegen/box/functions/kt2716.kt

@@ -1,6 +1,6 @@
 package someTest
 
-public class Some private(val v: String) {
+public class Some private constructor(val v: String) {
     companion object {
         public fun init(v: String): Some {
             return Some(v)

compiler/testData/codegen/box/secondaryConstructors/dataClasses.kt

@@ -12,7 +12,7 @@ data class A1(val prop1: String) {
     fun f(): String = "$prop1#$prop2#$prop3"
 }
 
-data class A2 private () {
+data class A2 private constructor() {
     var prop1: String = ""
     var prop2: String = "const2"
     var prop3: String = ""

compiler/testData/codegen/box/secondaryConstructors/superCallPrimary.kt

@@ -1,6 +1,6 @@
 var sideEffects: String = ""
 
-abstract class B protected(val arg: Int) {
+abstract class B protected constructor(val arg: Int) {
     val parentProp: String
     init {
         sideEffects += "zero#"

compiler/testData/codegen/boxWithStdlib/synthetic/syntheticAccessorNames.kt

@@ -2,7 +2,7 @@
 // This is crucial for some JVM frameworks like Quasar which rely on the bytecode being similar to the one generated by javac
 // See https://youtrack.jetbrains.com/issue/KT-6870
 
-class PrivateConstructor private() {
+class PrivateConstructor private constructor() {
     class Nested { val a = PrivateConstructor() }
 }
 

compiler/testData/codegen/defaultArguments/reflection/privateConstructor.kt

@@ -1,4 +1,4 @@
-class Foo private(val a: Int = 1) {}
+class Foo private constructor(val a: Int = 1) {}
 
 // CLASS: Foo
 // HAS_DEFAULT_CONSTRUCTOR: false