Libc++'s policy is to support only the latest released Xcode, which is
Xcode 16.x. We did update our CI jobs to Xcode 16.x, but we forgot to
update the documentation, which still mentioned Xcode 15. This patch
updates the documentation and cleans up outdated mentions of
apple-clang-15 in the test suite.
GCC 14 has been released a while ago. We've updated the CI to use GCC 14
now. This removes any old annotations in the tests and updates the
documentation to reflect the updated version requirements.
This is a first step towards splitting up the <__config> header. The
<__config> header is large and rather disorganized at this point,
leading to confusion and subtle mistakes. For example, we never noticed
that the string layout used on arm64 was only enabled for the Clang
compiler, as the setting being in the compiler == clang block was
probably never intentional.
The danger of splitting up the <__config> header is to implicitly use
undefined macros that should have been defined prior to their usage,
however this can be remediated with -Wundef and we've started moving
towards -Wundef enforceable macros.
