This was setting a bad example. DiagIDs are a limited resource and the message
argument is evaluated as a format string.

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for use in reporting diagnostics.
 - We don't want to use the Action's own CompilerInstance, because that is only
   initialized during file processing and I like that invariant.

Also, if ParseArgs returns false then abandon execution.

Also, remove unused PluginASTAction::PrintHelp virtual method.

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 - The build scriptage is about twice as long as the code, which is nice. :)

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