Fix ArgvToCommandLine SFINAE
          #536
        
          
      
                
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The TL;DR is that you can't SFINAE in type definitions, so we hit a hard compilation error during overload resolution when
ArgvToCommandLineis called with just an argc+argv pair with the intent of using the default flags. My original primary use-case used avector<basic_string>and the tests only called the argc+argv overload with flags explicitly set, so this never got caught.