std-coroutine: include for LLVM-14
std-coroutine: include for LLVM-14

since std::experimental::coroutine_traits will be removed in LLVM 15,
but are available in LLVM-14.

Update the check to prevent the warning for LLVM-14, as well..

This is a followup to 0a8eea3d7e9efba6fefb0ac783a8b988bb1feffb

Signed-off-by: Ben Pope <ben@redpanda.com>

Closes #1073

syuu1228/seastar,scylladb/seastar,syuu1228/seastar,avikivity/seastar,avikivity/seastar,scylladb/seastar,avikivity/seastar,syuu1228/seastar,scylladb/seastar