Previously TcTyCons were used only for knot-tying, but now they
are also used after an error, to add a benign TyCon to the envt
so we can carry on; see TyCon.makeRecoveryTyCon.  But since it
is used in this way, subsequent declarations may see a TcTyCon
(e.g. during injectivity checks) and should not have a heart attack
as a result.

See Note [TcTyCon] in TyCon.

This fixes Trac #13271