Fix panic when using pattern synonyms with DisambiguateRecordFields

This fixes a find_tycon panic when constructing a record pattern
synonym when DisambiguateRecordFields (turned on by RecordWildCards)
is enabled. The handling of record wild cards in such constructions
isn't completely satisfactory, but doing better will require the
Parent type to be more informative, as I'll explain on Trac #11228.

Test Plan: New test patsyn/should_compile/T11283.hs

Reviewers: mpickering, austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1695

GHC Trac Issues: Trac #11283