Recent lexer perf compile error with alex-3.5.1.0

Summary

This commit yields a Lexer.hs that doesn't compile when using alex-3.5.1.0 (prevailing version on Hackage). Errors look like

compiler/GHC/Parser/Lexer.x:2871:20: error: [GHC-88464]
    Data constructor not in scope: C# :: t0 -> Char
    Suggested fixes:
      • Perhaps use one of these:
          ‘GHC.Exts.C#’ (imported from GHC.Exts),
          ‘GHC.Exts.D#’ (imported from GHC.Exts),
          ‘GHC.Exts.F#’ (imported from GHC.Exts)
      • Perhaps you want to add ‘C#’ to the import list in the import of
        ‘GHC.Exts’