This PR fixes the failures currently being experienced on the Bitcoin Core appveyor CI job.
The cause of the failure is the appveyor image being updated to version Visual Studio 2019 16.4. From the v16.4 release notes at https://docs.microsoft.com/en-us/visualstudio/releases/2019/release-notes it's not clear why the nodiscard warning behaviour has changed.
I wonder if this solves all the problems, there was also some zmq.h / libevent.h problem at least in the other PR.
They zeromq and libevent errors could be related to the vcpkg dependencies. I had to reinstall my vcpkg packages after updating Visual Studio. The error I got was libraries being built with an older version of the runtime even though the latest version is still the same at v.1.4.2the object or library file was created with an older compiler than other objects; rebuild old objects and libraries.
Reinstalling the vcpkg dependencies fixes zeromq and libevent but now the Qt5 binaries are also out of date. Sigh. That'll take a bit of time to adjust.
Qt updated needed so will submit a new PR with more appropriate topic.