Change MISSING_CONSTRUCTOR_KEYWORD severity to error

And adjust testData

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() }
 }