It's confusing because if a function is unstable overall, there's no
need to highlight the constness is also unstable. Technically, these
attributes (overall stability and const-stability) are separate, but in
practice, we don't even show the const-unstable's feature flag (it's
normally the same as the overall function).

If a const function is unstable overall (and thus, in all circumstances
I know of, also const-unstable), we should show the option to use it as
const. You need to enable a feature to use the function at all anyway.

If the function is stabilized without also being const-stabilized, then
we do not show the const keyword and instead show "const: unstable" in
the version info.