- Treat protocol requirements just like associated types in how they
inherit usable-from-inline-ness, rather than special-casing them in
the check for inlinable code.
- Enum elements are already treated this way, so we can remove a
redundant check for that.
- Finally, start enforcing that 'dynamic' declarations need to be
'@usableFromInline' to be used in inlinable functions...in Swift 5
mode or resilient code. I didn't even add a warning in Swift 4/4.2
because it was illegal to use '@usableFromInline' on a 'dynamic'
declaration in Swift 4.2. (Oops.)
