Move warning flag handling into Flags module
I need this to make the Logger independent of DynFlags.
Also fix copy-paste errors: Opt_WarnNonCanonicalMonadInstances
was associated to "noncanonical-monadfail-instances" (MonadFailInstances vs
MonadInstances).
In the process I've also made the default name for each flag more
explicit.
This is preliminary work for #19877 (closed) (making systools independent of DynFlags).