Remove record field from Solo
Ticket #20562 (closed) revealed that Solo, which is a wired-in TyCon, had
a record field that wasn't being added to the type env. Why not?
Because wired-in TyCons don't have record fields.
It's not hard to change that, but it's tiresome for this one use-case,
and it seems easier simply to make getSolo into a standalone
function.
On the way I refactored the handling of Solo slightly, to put it
into wiredInTyCons (where it belongs) rather than only in
knownKeyNames.