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.
07a9dae406