[libc++][format] Update LWG3701 status. (#80545)

The issue has been resolved in https://reviews.llvm.org/D121138 since it
was needed to implement format. This updates the status of the LWG-issue
filed for this review.

Marks as complete:
- LWG3701 Make formatter<remove_cvref_t<const charT[N]>, charT>
requirement explicit