IR: mark interface delegate fields as synthetic

This requires separating them origin-wise from property delegates.

compiler/testData/ir/irText/declarations/annotations/delegateFieldWithAnnotations.txt

@@ -16,7 +16,7 @@ FILE fqName:<root> fileName:/delegateFieldWithAnnotations.kt
         public open fun toString (): kotlin.String [fake_override] declared in kotlin.Annotation
       $this: VALUE_PARAMETER name:<this> type:kotlin.Any
   PROPERTY name:test1 visibility:public modality:FINAL [delegated,val]
-    FIELD DELEGATE name:test1$delegate type:kotlin.Lazy<kotlin.Int> visibility:private [final,static]
+    FIELD PROPERTY_DELEGATE name:test1$delegate type:kotlin.Lazy<kotlin.Int> visibility:private [final,static]
       annotations:
         Ann
       EXPRESSION_BODY
@@ -33,6 +33,6 @@ FILE fqName:<root> fileName:/delegateFieldWithAnnotations.kt
         RETURN type=kotlin.Nothing from='public final fun <get-test1> (): kotlin.Int declared in <root>'
           CALL 'public final fun getValue <T> (thisRef: kotlin.Any?, property: kotlin.reflect.KProperty<*>): T of kotlin.getValue [inline,operator] declared in kotlin' type=kotlin.Int origin=null
             <T>: kotlin.Int
-            $receiver: GET_FIELD 'FIELD DELEGATE name:test1$delegate type:kotlin.Lazy<kotlin.Int> visibility:private [final,static]' type=kotlin.Lazy<kotlin.Int> origin=null
+            $receiver: GET_FIELD 'FIELD PROPERTY_DELEGATE name:test1$delegate type:kotlin.Lazy<kotlin.Int> visibility:private [final,static]' type=kotlin.Lazy<kotlin.Int> origin=null
             thisRef: CONST Null type=kotlin.Nothing? value=null
             property: PROPERTY_REFERENCE 'public final test1: kotlin.Int [delegated,val]' field=null getter='public final fun <get-test1> (): kotlin.Int declared in <root>' setter=null type=kotlin.reflect.KProperty0<kotlin.Int> origin=PROPERTY_REFERENCE_FOR_DELEGATE