[Bug libstdc++/80448] #include fails with Clang 5.0

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80448

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Apparently related to CWG1351 <https://wg21.link/cwg1351> and solved by
removing 'noexcept' from the ctor.