This reverts f5b275a2
and changes the places that looked for `Lit (MachStr _))`
to use `exprIsMbTickedLitString_maybe` to unwrap ticks as
necessary.
Also updated relevant comments.

Test Plan:
I added 3 new tests that previously reproduced.
GHC HEAD now builds with -g

Reviewers: simonpj, simonmar, bgamari, hvr, goldfire

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #14779

Differential Revision: https://phabricator.haskell.org/D4470