This should be a trivial NFC.
It's worth remarking that historically libc++ includes <__config> before all other headers (in case some header depends on <__config> macros). However, if all our headers include <__config>, then it doesn't really matter — maybe we include <__bit> first, but then <__bit> includes <__config>, so it all works out.
I'm not sure what's going on with the errors in https://buildkite.com/llvm-project/libcxx-ci/builds/3355 (on the Apple builds only).