Revert "Refactor CallStack defaulting slightly"
Revert "Refactor CallStack defaulting slightly"

This reverts commit 317236db308d9bd497a11fa4c455428fc7477405.

I totally missed that in simplifyInfer for local functions
we must NOT default call stacks.  So I'm reverting this.

Fortunately caught by T10845, which sadly isn't run by
validate --fast

ezyang/ghc,shlevy/ghc,olsner/ghc,mettekou/ghc,shlevy/ghc,mettekou/ghc,sdiehl/ghc,olsner/ghc,olsner/ghc,ezyang/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,mettekou/ghc,shlevy/ghc,sdiehl/ghc,mettekou/ghc,olsner/ghc,olsner/ghc,sdiehl/ghc,olsner/ghc,sdiehl/ghc,mettekou/ghc,sdiehl/ghc,shlevy/ghc,olsner/ghc,ezyang/ghc,ezyang/ghc,mettekou/ghc,shlevy/ghc,mettekou/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc