Enable -Wstar-is-type by default (#22759)
Following the plan in GHC Proposal #143 "Remove the * kind syntax", which states:
In the next release (or 3 years in), enable
-fwarn-star-is-type
by default.
The "next release" happens to be 9.6.1.
I also moved the T21583
test case from should_fail
to should_compile
, because the only reason it was failing was -Werror=compat
in our test suite configuration.