[FIR] Set vararg modifier on data class copy method parameters

This only happens in red code, but it prevents unhelpful initializer
type mismatch errors.

analysis/low-level-api-fir/src/org/jetbrains/kotlin/analysis/low/level/api/fir/stubBased/deserialization/StubBasedClassDeserialization.kt

@@ -212,6 +212,7 @@ internal fun deserializeClassToSymbol(
                     },
                     toFirSource = { src, kind -> KtFakeSourceElement(src as PsiElement, kind) },
                     addValueParameterAnnotations = { annotations += context.annotationDeserializer.loadAnnotations(it) },
+                    isVararg = { it.isVarArg }
                 )
             )
         }