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.

Test Plan: new test th/T12130

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2321

GHC Trac Issues: #12130

(cherry picked from commit 4d71cc89b4e9648f3fbb29c8fcd25d725616e265)