DebugInfo: Forward HandleTagDeclRequiredDefinition through MultiplexConsumer to fix debug info emission in the presence of plugins.

When plugins are used the Multiplex(AST)Consumer is employed to dispatch
to both the plugin ASTConsumers and the IRGen ASTConsumer. It wasn't
dispatching a critical call for debug info, resulting in plugin users
having a negative debugging experience.

While I'm here, forward a bunch of other missing calls through the
consumer that seem like they should be there.

To test this, use the example plugin (requires plugins and examples) and
split the test case up so that the plugin testing can be done under that
requirement while the non-plugin testing will execute even in builds
that don't include plugin support or examples.