since std::experimental::coroutine_traits will be removed in LLVM 15,
let's prepare for this change by including <coroutine> when compiling
clang-15 and up.

without this change, when compiling an application using Seastar
coroutine, we could run into following warning:

/home/kefu/dev/redpanda/src/v/net/server.cc:141:19: error: support for std::experimental::coroutine_traits will be removed in LLVM 15; use std::coroutine_traits instead [-Werror,-Wdeprecated-experimental-coroutine]
                  co_return ss::stop_iteration::yes;
                  ^
/home/kefu/dev/redpanda/build/deps_install/include/seastar/core/std-coroutine.hh:124:7: note: 'coroutine_traits' declared here
class coroutine_traits { };
      ^
1 error generated.

after this change, the warning is gone.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>
Message-Id: <20220417023709.2387661-1-tchaikov@gmail.com>
(cherry picked from commit 0a8eea3)

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 0a8eea3

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

Closes scylladb#1073

(cherry picked from commit 070ab10)