Add warning for initializers with obsolete syntax

Expected 'init' keyword before class initializer

compiler/testData/diagnostics/tests/variance/privateToThis/FunctionCall.kt

@@ -7,7 +7,7 @@ class Test<in I> {
 
     fun apply(<!UNUSED_PARAMETER!>i<!>: I) {}
 
-    {
+    init {
         foo()
         this.foo()
     }

compiler/testData/diagnostics/tests/variance/privateToThis/GetVal.kt

@@ -4,7 +4,7 @@ fun <T, R> with(receiver: T, f: T.() -> R): R = receiver.f()
 class Test<in I, out O> {
     private val i: I = getT()
 
-    ;{
+    init {
         apply(i)
         apply(this.i)
     }

compiler/testData/diagnostics/tests/variance/privateToThis/SetVar.kt

@@ -5,7 +5,7 @@ class Test<in I, out O> {
     private var i: I = getT()
     private val j: I
 
-    ;{
+    init {
         j = getT()
         i = getT()
         this.i = getT()

compiler/testData/diagnostics/tests/variance/privateToThis/ValReassigned.kt

@@ -3,7 +3,7 @@ fun <T> getT(): T = null!!
 class A<in I>(init: I) {
     private val i: I
 
-    {
+    init {
         i = getT()
     }
 
@@ -12,7 +12,7 @@ class A<in I>(init: I) {
 
     private var i4 = getT<I>()
 
-    ;{
+    init {
         i2 = getT()
         i3 = init
         i4 = i3