Clang 17 will land with unaddressed coroutine bugs, see
It would be useful to have a way to use other C++20, but guarantee
coroutines are not used. This patch achieves it with a warning that
fires on co_await, co_return and co_yield.
The new warning is -Wpre-c++20-compat-coroutines, which is also
enabled with -Wpre-c++20-compat-pedantic.