Summary:
#10816 surfaced because we were renaming top-level fixity
declarations with a different code path (`rnSrcFixityDecl`) than
the code path for fixity declarations inside of type classes, which
is not privy to names that exist in the type namespace. Luckily, the
fix is simple: use `rnSrcFixityDecl` in both places.

Test Plan: make test TEST=T10816

Reviewers: austin, bgamari, simonpj

Reviewed By: simonpj

Subscribers: simonpj, rwbarton, thomie

GHC Trac Issues: #10816

Differential Revision: https://phabricator.haskell.org/D4077