patch applied (ghc): Adding a GENERATED pragma

[Andy Gill]

>
> LINE can already refer to an external non-.hs file. This is exactly  
> how
> things works now with all manner of pre-processors, alex, happy,  
> cpphs,
> c2hs, hsc2hs, etc.
>
> I don't quite have an intuition for this yet, can you give an  
> example of
> how this more fine grained pragma works and some use-case not  
> currently
> covered by LINE?
>

Ahhh. LINE does? Oh.

- GENERATED works on an expression based level, and can specify  
specific spans.
   (20:4 - 21:8, for example).
  - GENERATED is a true pragma; it hints where things come from, but  
does not change
   the error message. I want the Haskell source spans to remain  
unchanges.

But perhaps a variant of LINE would be a better name.

Andy