Improve ManagedStreams (#7367)

Improve ManagedStreams (#7367)

This PR addresses #7361, but also a more general issue about regular-output-channel being able to hold stderr and diagnostic-output-channel being able to hold stdout.
It therefore changes how non-owned streams are stored: beforehand, an explicit stream would always be owned by the ManagedStream (through a std::shared_ptr) and a nullptr implicitly stood for stdout, stderr or stdin. Now we explicitly hold a pointer to a non-owned stream for these special values.

Fixes #7361.