FIR2IR: change backing field visibility to private

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

@@ -7,7 +7,7 @@ FILE fqName:<root> fileName:/inlineClass.kt
         DELEGATING_CONSTRUCTOR_CALL 'public constructor <init> () [primary] declared in kotlin.Any'
         INSTANCE_INITIALIZER_CALL classDescriptor='CLASS CLASS name:Test modality:FINAL visibility:public [inline] superTypes:[kotlin.Any]'
     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>.Test.<init>' type=kotlin.Int origin=INITIALIZE_PROPERTY_FROM_PARAMETER
       FUN DEFAULT_PROPERTY_ACCESSOR name:<get-x> visibility:public modality:FINAL <> ($this:<root>.Test) returnType:kotlin.Int
@@ -15,7 +15,7 @@ FILE fqName:<root> fileName:/inlineClass.kt
         $this: VALUE_PARAMETER name:<this> type:<root>.Test
         BLOCK_BODY
           RETURN type=kotlin.Nothing from='public final fun <get-x> (): kotlin.Int declared in <root>.Test'
-            GET_FIELD 'FIELD PROPERTY_BACKING_FIELD name:x type:kotlin.Int visibility:public [final]' type=kotlin.Int origin=null
+            GET_FIELD 'FIELD PROPERTY_BACKING_FIELD name:x type:kotlin.Int visibility:private [final]' type=kotlin.Int origin=null
               receiver: GET_VAR '<this>: <root>.Test declared in <root>.Test.<get-x>' type=<root>.Test origin=null
     FUN FAKE_OVERRIDE name:equals visibility:public modality:OPEN <> ($this:kotlin.Any, other:kotlin.Any?) returnType:kotlin.Boolean
       overridden:
@@ -30,3 +30,4 @@ FILE fqName:<root> fileName:/inlineClass.kt
       overridden:
         public open fun toString (): kotlin.String declared in kotlin.Any
       $this: VALUE_PARAMETER name:<this> type:kotlin.Any
+