niteria authored Jan 13, 2016 and Ben Gamari committed Jan 13, 2016

This aids with debugging, since all you have to do to get more
stack frames is add a constraint `(?callStack :: CallStack) =>`.

Old output:
```
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.1.20160107 for x86_64-unknown-linux):
        ASSERT failed!
  file compiler/types/TyCoRep.hs line 1800
  InScope []
  [Xuv :-> n_av5[sk]]
  []
```

New output:
```
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.1.20160107 for x86_64-unknown-linux):
        ASSERT failed!
  CallStack (from ImplicitParams):
  assertPprPanic, called at compiler/types/TyCoRep.hs:1800:95 in
ghc:TyCoRep
  InScope []
  [Xuv :-> n_av5[sk]]
  []
```

Test Plan:
harbormaster
manual testing

Reviewers: austin, gridaphobe, bgamari

Reviewed By: gridaphobe, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1751