FIR2IR: use IR_EXTERNAL_DECLARATION_STUB for external classes

compiler/testData/ir/irText/stubs/constFromBuiltins.fir.txt

@@ -1,11 +0,0 @@
-FILE fqName:<root> fileName:/constFromBuiltins.kt
-  PROPERTY name:test visibility:public modality:FINAL [val]
-    FIELD PROPERTY_BACKING_FIELD name:test type:kotlin.Int visibility:private [final,static]
-      EXPRESSION_BODY
-        CALL 'public final fun <get-MIN_VALUE> (): kotlin.Int declared in kotlin.Int.Companion' type=kotlin.Int origin=GET_PROPERTY
-          $this: GET_OBJECT 'CLASS OBJECT name:Companion modality:FINAL visibility:public [companion] superTypes:[kotlin.Any]' type=kotlin.Int.Companion
-    FUN DEFAULT_PROPERTY_ACCESSOR name:<get-test> visibility:public modality:FINAL <> () returnType:kotlin.Int
-      correspondingProperty: PROPERTY name:test visibility:public modality:FINAL [val]
-      BLOCK_BODY
-        RETURN type=kotlin.Nothing from='public final fun <get-test> (): kotlin.Int declared in <root>'
-          GET_FIELD 'FIELD PROPERTY_BACKING_FIELD name:test type:kotlin.Int visibility:private [final,static]' type=kotlin.Int origin=null

compiler/testData/ir/irText/stubs/constFromBuiltins.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // DUMP_EXTERNAL_CLASS kotlin.Int
 
 val test = Int.MIN_VALUE