AppleClang 15 was released on September 18th and is now stable. Per our
policy, we're bumping the supported AppleClang compiler to the latest
release. This allows cleaning up the test suite, but most importantly
unblocking various other patches that are blocked on bumping the
compiler requirements.
Concepts should be supported by all supported compilers. This is
probably a left over from an older version of the expected patch.
Reviewed By: #libc, philnik
Differential Revision: https://reviews.llvm.org/D140632