Summary:
So that assertion failures have full call stack information attached
when using `ghc -fexternal-interpreter -prof`.  Here's one I just
collected by inserting a dummy assert in Happy:

```
*** Exception: Assertion failed
CallStack (from ImplicitParams):
  assert, called at ./First.lhs:37:11 in main:First
CallStack (from -prof):
  First.mkFirst (First.lhs:37:11-27)
  First.mkFirst (First.lhs:37:11-93)
  Main.main2.runParserGen.first (Main.lhs:107:48-56)
  Main.main2.runParserGen.first (Main.lhs:107:27-57)
  Main.main2.runParserGen (Main.lhs:(96,9)-(276,9))
  Main.main2.runParserGen (Main.lhs:(90,9)-(276,10))
  Main.main2.runParserGen (Main.lhs:(86,9)-(276,10))
  Main.main2.runParserGen (Main.lhs:(85,9)-(276,10))
  Main.main2 (Main.lhs:74:20-43)
  Main.main2 (Main.lhs:(64,9)-(78,61))
  Main.main (Main.lhs:57:9-18)
```

Test Plan: validate

Reviewers: erikd, hvr, austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1765

GHC Trac Issues: #11047