Unwrap function before calling IsConstructor
`IsConstructor` returns true for all wrappers that are callable. Wrapped
arrow functions are callable and thus `IsConstructor` will return true
if given one; however, if we unwrap the arrow function, `IsConstructor`
will return false. The latter is the correct behavior here.
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).
<!-- Either: -->
- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____
<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->
<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/17250)
<!-- Reviewable:end -->
IsConstructor returns true for all wrappers that are callable. Wrapped
arrow functions are callable and thus IsConstructor will return true
if given one; however, if we unwrap the arrow function, IsConstructor
will return false. The latter is the correct behavior here.
