KT-56687: Unify handling of EnumEntries.

`supportsEnumEntries` in `EnumCLassLowering` is not really
needed, but it was left just in case.
Both K1 and K2 ensure the feature is
enabled before they generate IR.

`supportsEnumEntries` is not enough, because the frontend will not
generate `entries` if `kotlin.enums.EnumEntries` is not available
(linking against an outdated stdlib is considered a valid use-case).

From the resolution point of view,
it's OK to resolve into `entries` while
the feature is off, because then K1 reports an error on the call site.

^KT-56687 Fixed
^KT-55614 Fixed

compiler/ir/backend.jvm/lower/src/org/jetbrains/kotlin/backend/jvm/lower/EnumClassLowering.kt

@@ -124,7 +124,13 @@ private class EnumClassLowering(private val context: JvmBackendContext) : ClassL
             val valuesField = buildValuesField(valuesHelperFunction)
 
             val entriesField = when {
-                irClass.declarations.any { it.isGetEntriesFunction } -> {
+                !irClass.hasGetEntriesFunction -> {
+                    null
+                }
+                !supportsEnumEntries -> {
+                    error("The frontend must have checked if the feature is supported while emitting the IR")
+                }
+                else -> {
                     // Constructs the synthetic $entries() function that returns plain $VALUES without copy
                     val entriesHelperFunction = buildEntriesHelperFunction(valuesField)
 
@@ -137,12 +143,6 @@ private class EnumClassLowering(private val context: JvmBackendContext) : ClassL
                      */
                     buildEntriesField(entriesHelperFunction)
                 }
-                supportsEnumEntries -> {
-                    error("kotlin.enums.EnumEntries is not available to the frontend. Is non-full kotlin stdlib being used?")
-                }
-                else -> {
-                    null
-                }
             }
 
             // Add synthetic parameters to enum constructors and implement the values and valueOf functions
@@ -152,6 +152,9 @@ private class EnumClassLowering(private val context: JvmBackendContext) : ClassL
             irClass.transformChildrenVoid(EnumClassCallTransformer())
         }
 
+        private val IrClass.hasGetEntriesFunction: Boolean
+            get() = declarations.any { it.isGetEntriesFunction }
+
         private val IrDeclaration.isGetEntriesFunction: Boolean
             get() = this is IrFunction && name == SpecialNames.ENUM_GET_ENTRIES && origin == IrDeclarationOrigin.ENUM_CLASS_SPECIAL_MEMBER