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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@110039 91177308-0d34-0410-b5e6-96231b3b80d8