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 #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: #11283