Make Warns = Messages DriverMessage

Resolves #23261 (closed)

After splitting data DynFlags into own module, we can change command line parsing to produce DriverMessages. As a result e.g. ignored extension due enabled Safe Haskell have a GHC-error code.

The new DriverMessages have WarningWithoutFlag as their reason. But that can be changed for individual flags. cc @adamgundry}