Apply changes to match AST

Summary: Match the changes to the AST made in D426

Test Plan: built with GHC

Differential Revision: https://phabricator.haskell.org/D494

GHC Trac Issues: #9628
Replace head with case/panic
Merge remote-tracking branch 'haskell/wip/ast-prepare-annotations-final3' into wip/ast-prepare-annotations-final3