Warn if compartment call returns void

[rmn30]

It would be good if clang would warn if the return type of a compartment call is void. Compartment calls can fail unexpectedly and return -1 (e.g. due to trusted stack exhaustion). Callers should be aware of and check for this, therefore it is never appropriate for a compartment call to return void. For the same reason we should also consider automatically adding the nodiscard attribute to compartment calls.