FIR: introduce diagnostic NESTED_CLASS_NOT_ALLOWED

compiler/testData/diagnostics/tests/enum/inner/insideInnerClassNotAllowed.fir.kt

@@ -1,7 +0,0 @@
-class A {
-    inner class B {
-        enum class E {
-            ENTRY
-        }
-    }
-}

compiler/testData/diagnostics/tests/enum/inner/insideInnerClassNotAllowed.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 class A {
     inner class B {
         <!NESTED_CLASS_NOT_ALLOWED!>enum class E<!> {

compiler/testData/diagnostics/tests/enum/inner/twoEnumsInClassObjectAndInnerClass.fir.kt

@@ -1,9 +0,0 @@
-class A {
-    companion object {
-        enum class E { ENTRY }  // OK
-    }
-    
-    inner class B {
-        enum class E { ENTRY }
-    }
-}

compiler/testData/diagnostics/tests/enum/inner/twoEnumsInClassObjectAndInnerClass.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 class A {
     companion object {
         enum class E { ENTRY }  // OK