[FIR] Create checker for simple enum entries with missing overrides

Complex enum entries (those with bodies) with missing overrides
are covered by an existing checker that checks all types of classes. But
simple enum entries (those without bodies) were not covered and require
a separate checker for missing overrides.

#KT-58637 Fixed

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

@@ -2,7 +2,7 @@ package abstract
 
 
 enum class MyEnum() {
-    INSTANCE;
+    <!ABSTRACT_MEMBER_NOT_IMPLEMENTED_BY_ENUM_ENTRY!>INSTANCE<!>;
     //properties
     <!MUST_BE_INITIALIZED_OR_BE_ABSTRACT!>val a: Int<!>
     val a1: Int = 1