AppleClang 1403 has some bugs that prevent std::source_location from
working properly on it. Consequently, we XFAILed the unit test for
source_location with that compiler. However, we should also avoid
advertising that the feature is supported on that compiler, otherwise
our feature-test macros lie. This was noticed to break Boost.Asio
when building with a recent libc++ and AppleClang 14.0.3.