and implement it for @isolated(any) function types.
The existing testing was pretty broken: we were diagnosing all sorts
of things that don't require type metadata (like using a tuple with
an extended existential in a value position in an API signature)
and not diagnosing several things that do (like covariant function
conversions that erase types). There's therefore some risk to this
patch, but I'm not too worried because needing metadata like this is
pretty uncommon, and it's likely that programs won't build correctly
anyway --- it'll just get caught by the linker instead of the compiler.
