We parse `~Copyable` in an inheritance clause of enum and
struct decls as a synonym for the `@_moveOnly` attribute
being added to that decl. This completely side-steps the
additional infrastructure for generalized suppressed
conformances in favor of a minimal solution. One benefit of
this minimal solution is that it doesn't risk introducing
any back-compat issues with older compilers or stdlibs.
The trade-off is that we're more committed to supporting
`@_moveOnly` in compiled modules in the future. In fact,
this change does not deprecate `@_moveOnly` in any way.
resolves rdar://106775103
I enabled move-only types by default, but I didn't
realize that the `Feature::MoveOnly` needs to graduate
into a `LANGUAGE_FEATURE` so that `EvaluateIfConfigCondition`
will recognize `$MoveOnly` as being true.
fixes rdar://107050387
