[Bug c++/88075] [feature-request] allow "concept" instead of "concept bool" with -fconcepts

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88075

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
GCC 7 doesn't support the -std=c++2a option, so it doesn't seem necessary to
backport it to GCC 7.