Revert "Remove kind generalisation from tcRnType"

Authored by goldfire on Oct 29 2018, 11:15 AM.

Description

Revert "Remove kind generalisation from tcRnType"

This reverts commit 3a51abd04432ea3d13e4ea3c5a592f038bd57432.

I had hit the wrong button when trying to validate the original
commit... and ended up committing it prematurely instead.
This reversion commit
also updates the comments to explain why we kind-generalise.