Fixes https://github.com/llvm/llvm-project/issues/104307
This patch implements LWG3476 by removing the incorrect decay-copy in
std::async. The decay-copy was being applied twice, once explicitly via
_LIBCPP_AUTO_CAST and once in __async_func's tuple constructor.
(https://github.com/llvm/llvm-project/issues/143828)
It also adds static_assert mandates to std::thread and std::async (which
were already implicitly enforced) and expands test coverage.
Apple dropped support for some older platforms, so we can also remove
the annotations for them.
See https://developer.apple.com/support/xcode/ for the supported
versions.
This patch adds a large number of missing includes in the libc++ headers
and the test suite. Those were found as part of the effort to move
towards a mostly monolithic top-level std module.
