[FIR] Implement CLASS_CANNOT_BE_EXTENDED_DIRECTLY

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

@@ -1,9 +0,0 @@
-class Test1 : Enum<Test1>("", 0)
-
-class Outer {
-    class Test2 : Enum<Test2>("", 0)
-}
-
-fun outer() {
-    class Test3 : Enum<Test3>("", 0)
-}

compiler/testData/diagnostics/tests/enum/extendingEnumDirectly.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 class Test1 : <!CLASS_CANNOT_BE_EXTENDED_DIRECTLY!>Enum<Test1><!>("", 0)
 
 class Outer {