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
     }