FIR2IR: change backing field visibility to private

compiler/testData/ir/irText/classes/enumClassModality.fir.txt

@@ -56,7 +56,7 @@ FILE fqName:<root> fileName:/enumClassModality.kt
           <E>: <none>
         INSTANCE_INITIALIZER_CALL classDescriptor='CLASS ENUM_CLASS name:TestFinalEnum2 modality:FINAL visibility:public superTypes:[kotlin.Enum]'
     PROPERTY name:x visibility:public modality:FINAL [val]
-      FIELD PROPERTY_BACKING_FIELD name:x type:kotlin.Int visibility:public [final]
+      FIELD PROPERTY_BACKING_FIELD name:x type:kotlin.Int visibility:private [final]
         EXPRESSION_BODY
           GET_VAR 'x: kotlin.Int declared in <root>.TestFinalEnum2.<init>' type=kotlin.Int origin=INITIALIZE_PROPERTY_FROM_PARAMETER
       FUN DEFAULT_PROPERTY_ACCESSOR name:<get-x> visibility:public modality:FINAL <> ($this:<root>.TestFinalEnum2) returnType:kotlin.Int
@@ -64,7 +64,7 @@ FILE fqName:<root> fileName:/enumClassModality.kt
         $this: VALUE_PARAMETER name:<this> type:<root>.TestFinalEnum2
         BLOCK_BODY
           RETURN type=kotlin.Nothing from='public final fun <get-x> (): kotlin.Int declared in <root>.TestFinalEnum2'
-            GET_FIELD 'FIELD PROPERTY_BACKING_FIELD name:x type:kotlin.Int visibility:public [final]' type=kotlin.Int origin=null
+            GET_FIELD 'FIELD PROPERTY_BACKING_FIELD name:x type:kotlin.Int visibility:private [final]' type=kotlin.Int origin=null
               receiver: GET_VAR '<this>: <root>.TestFinalEnum2 declared in <root>.TestFinalEnum2.<get-x>' type=<root>.TestFinalEnum2 origin=null
     CLASS ENUM_ENTRY name:X1 modality:FINAL visibility:public superTypes:[<root>.TestFinalEnum2]
       $this: VALUE_PARAMETER INSTANCE_RECEIVER name:<this> type:<root>.TestFinalEnum2.X1
@@ -387,3 +387,4 @@ FILE fqName:<root> fileName:/enumClassModality.kt
       overridden:
         public open fun toString (): kotlin.String declared in kotlin.Any
       $this: VALUE_PARAMETER name:<this> type:kotlin.Any
+