Note [SPECIALISE pragmas for imported Ids]

An imported Id may or may not have an unfolding.  If not, we obviously
can't specialise it here; indeed the desugar falls over (#18118).

We used to test whether it had a user-specified INLINABLE pragma but,
because of Note [Worker/wrapper for INLINABLE functions] in
GHC.Core.Opt.WorkWrap, even an INLINABLE function may end up with
a wrapper that has no pragma, just an unfolding (#19246).  So now
we just test whether the function has an unfolding.

There's a risk that a pragma-free function may have an unfolding now
(because it is fairly small), and then gets a bit bigger, and no
longer has an unfolding in the future.  But then you'll get a helpful
error message suggesting an INLINABLE pragma, which you can follow.
That seems enough for now.