[Bug libstdc++/81468] is_constructible gives the wrong answer for time_point construction

[daniel.kruegler at googlemail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81468

--- Comment #2 from Daniel Krügler <daniel.kruegler at googlemail dot com> ---
Shouldn't add a DR-1177 tag? (I forgot the exact construction pattern for this)
This may also help to validate that all other wording changes by this issue had
been implemented.