This patch fixes Trac #15805, where we found that
TcType.anyRewritableTyVar has one wrong case.
Besides the fix, it also:
- removed some unnecessary ASSERT2(tcIsTcTyVar...) in TcType, as now we have tcIsTcTyVar = isTyVar.
- fixed some comments
Test Plan: ./validate
Reviewers: goldfire, simonpj, bgamari
Reviewed By: simonpj
Subscribers: rwbarton, carter
GHC Trac Issues: Trac #15805
Differential Revision: https://phabricator.haskell.org/D5263