Fix slash escaping in cwrapper.c
Fix slash escaping in cwrapper.c

Summary: Escape `\` in paths on Windows in `cwapper.c` when we re-output the paths.

Test Plan: ./validate

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie

GHC Trac Issues: #13666

Differential Revision: https://phabricator.haskell.org/D3628

sdiehl/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,ezyang/ghc