Example warning:
cppdap/include/dap/future.h:172:14: error: implicitly-declared ‘dap::ResponseOrError<dap::StackTraceResponse>& dap::ResponseOrError<dap::StackTraceResponse>::operator=(const dap::ResponseOrError<dap::StackTraceResponse>&)’ is deprecated [-Werror=deprecated-copy]
Add missing constructors and assignment operators.
This probably adds more than is absolutely necessary, but there's no
harm in being thorough.
The `body` field of the Response is optional. Do not error if it is missing.
If the typeinfo of the response. This was incorrectly using the Request type.
Authored by kuafuwang, squashed by ben-clayton.