Test for type synonym loops on TyCon.
Summary: Previously, we tested for type synonym loops by doing a syntactic test on the literal type synonym declarations. However, in some cases, loops could go through hs-boot files, leading to an infinite loop (#12042); a similar situation can occur when signature merging. This commit replaces the syntactic test with a test on TyCon, simply by walking down all type synonyms until we bottom out, or find we've looped back. It's a lot simpler. Signed-off-by: Edward Z. Yang <firstname.lastname@example.org> Test Plan: validate Reviewers: simonpj, austin, bgamari Subscribers: goldfire, thomie Differential Revision: https://phabricator.haskell.org/D2656 GHC Trac Issues: #12042