libc++ eagerly instantiates the unique_ptr<APIConnection> destructor
when std::array<std::unique_ptr<APIConnection>, N> is parsed, requiring
sizeof(APIConnection). api_server.h only forward-declares APIConnection
(via list_entities.h), so the destructor instantiation fails for any
translation unit that includes api_server.h without also including
api_connection.h first.
Wrap the unique_ptr in a custom deleter (APIConnectionDeleter) whose
operator() is defined out-of-line in api_server.cpp where APIConnection
is complete. The default_delete<APIConnection> path is never
instantiated, so libc++'s incomplete-type assertion is avoided.
GCC/libstdc++ already deferred this instantiation, so this only affects
macOS host-platform builds (used by integration tests).
acc15ff495