Add warning for initializers with obsolete syntax

Expected 'init' keyword before class initializer

compiler/testData/diagnostics/tests/dataFlow/local/LocalClassInMemberOfLocalClass.kt

@@ -5,7 +5,7 @@ fun test(x: Any) {
     fun foo(y: Any) {
       if (y !is String) return
       class Local {
-        {
+        init {
           <!DEBUG_INFO_SMARTCAST!>x<!>.length()
           <!DEBUG_INFO_SMARTCAST!>y<!>.length()
         }

compiler/testData/diagnostics/tests/dataFlow/local/LocalClassInitializer.kt

@@ -3,7 +3,7 @@
 fun f(a: Any?) {
   if (a is B) {
     class C : X(<!DEBUG_INFO_SMARTCAST!>a<!>) {
-      {
+      init {
         <!DEBUG_INFO_SMARTCAST!>a<!>.foo()
       }
     }

compiler/testData/diagnostics/tests/dataFlow/local/NestedLocalClass.kt

@@ -3,7 +3,7 @@ fun test(x: Any) {
 
   class LocalOuter {
     inner class Local {
-      {
+      init {
         <!DEBUG_INFO_SMARTCAST!>x<!>.length()
       }
     }