fix breaking primitive declaration with attributes

[IwanKaramazow]

fixes #1639

/* before */
[@bs.val] external imul : (int, int) => int =
  "Math.imul";

/* after */
[@bs.val] external imul : (int, int) => int = "Math.imul";

[chenglou]

Aha, so did I diagnose right? Is it because it thought attributes were part of "first part"?

Ping me if you think it's ready to merge

[jordwalke]

This is exactly how I'd want it to break.
Next, imagine if we drop the [ ] for "simple enough" attributes:

@bs.val
external imul : (int, int) => int = "Math.imul";

This breaking pattern makes a lot of sense in that case.

[IwanKaramazow]

@chenglou Indeed, attributes where attached to the first part (all stuff up to =), which resulted in the printing going the wrong road.