Avoid find_tycon panic if datacon is not in scope

Avoid find_tycon panic if datacon is not in scope

When using TH to splice expressions involving record field construction,
the parent datacon may not be in scope.  We shouldn't panic about this,
because we will be renaming Exact RdrNames which don't require any
disambiguation.

Reviewers: austin, bgamari
Reviewed By: bgamari
Differential Revision: https://phabricator.haskell.org/D2321
GHC Trac Issues: #12130

Origin: upstream, https://git.haskell.org/ghc.git/commitdiff/694e0f3a08030

Gbp-Pq: Name find-tycon-panic