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	Fix the CAF info field of error Ids
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A bizarre bug.   In MkId, we build the Id for various error-y
Ids (like pAT_ERROR_ID) that we grab out of thin air in various
places (like the desugarer).  They were marked as not referring
to any CAFs, but this was a lie!  In fact, they refer to 'untangle'
(see GHC.Err) and thence to a CAF.

Result: GC crash under very obscure circumstances.  (Rob's optimistic
evaluator tickled it.)

Solution: give them more conservative IdInfo.

Two other better solutions to think about:

* Don't grab them out of thin air; instead get them from
  an interface file.

* Treat them as always-live (requires mod to garbage collector)
  so they don't need to be mentioned in SRTs at all