The Coverity compiler doesn't support C++2a yet, and thus doesn't
even recognize concept keywords. To allow serenity to be built and
analyzed on such compilers, add a fallback underdef to perform
the same template restriction based on AK::EnableIf<..> meta
programming.
Note: Coverity does seem to (annoyingly) define __cpp_concepts, even
though it doesn't support them, so we need to further check for
__COVERITY__ explicitly.