Change how preview=in is detected

This is a minor internal change, but will allow us to deprecate in ref at the parser level.

std/format/internal/write.d

@@ -1829,7 +1829,7 @@ enum HasToStringResult
     customPutWriterFormatSpec,
 }
 
-private enum hasPreviewIn = !is(typeof(mixin(q{(in ref int a) => a})));
+private enum hasPreviewIn = ((in int[256] a) => __traits(isRef, a))(int[256].init);
 
 template hasToString(T, Char)
 {