If the instance template is not marked as undetectable, we can
allow it to be called as a constructor. This broke previously with
commit ff056334.

R=verwaest@chromium.org

Bug: v8:7670
Change-Id: I6ecde33bd7532bea4786b2282efce9060bb76276
Reviewed-on: https://chromium-review.googlesource.com/c/1272579Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56577}