Frontend: Change PluginASTAction::ParseArgs to take a CompilerInstance object

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.

llvm-svn: 110039