Replace some FIR syntax errors with more proper diagnostics

compiler/testData/diagnostics/tests/enum/NonPrivateConstructor.fir.kt

@@ -1,7 +1,7 @@
-enum class E public constructor(val x: Int) {
+enum class E <!NON_PRIVATE_CONSTRUCTOR_IN_ENUM!>public constructor(val x: Int)<!> {
     FIRST();
 
-    internal constructor(): this(42)
+    <!NON_PRIVATE_CONSTRUCTOR_IN_ENUM!>internal constructor(): this(42)<!>
 
     constructor(y: Int, z: Int): this(y + z)
 }