In Exitify, zap idInfo of abstracted variables (fixes #15005)

as helpfully outlined by SPJ.

This commit copies a small bit code from `SetLevels` which could
reasonably be put in `Id` as `zapAllIdinfo`; I did not do this to make
merging this commmit into `ghc-8.4` easier.

If this commit gets merged, then presumably after commit
3f59d380 (test case) and
ae0cff0a (other fixes to Exitify.hs).

Differential Revision: https://phabricator.haskell.org/D4582

(cherry picked from commit 3cfb12d8)