Breakage due to CallStack refactoring

Hi Simon, In 317236db308d you performed some refactoring of CallStack defaulting and noted that there was no change in visisble behavior. However, it isn't seem that this is true. The following two tests [1] now fail as callstacks are no longer produced in their output, TEST="assert T10845" Presumably this isn't expected. Cheers, - Ben [1] https://phabricator.haskell.org/harbormaster/build/14964/?l=100