patch applied (ghc): Fix error reporting for contexts during deriving (Trac 958)

[Simon Peyton Jones]

Wed Nov  1 04:21:20 PST 2006  simonpj at microsoft.com
  * Fix error reporting for contexts during deriving (Trac 958)
  
  When doing the fixpoint iteration for 'deriving' we have to be careful
  not to end up in a loop, even if we have -fallow-undecidable-instances.
  
  Test is tcfail169
  

    M ./compiler/typecheck/TcDeriv.lhs -9 +4
    M ./compiler/typecheck/TcMType.lhs -15 +16
    M ./compiler/typecheck/TcSimplify.lhs -10 +37