Textual SIL was sometimes ambiguous when SILDeclRefs were used, because the textual representation of SILDeclRefs was the same for functions that have the same name, but different signatures.
* Private members may not satisfy protocol requirements, ever.
...because by construction they can be invoked from outside of the
type.
Finishing up SE-0025 ('private' and 'fileprivate').
* Update docs and mark SE-0025 ('private' and 'fileprivate') as done!
There's still improvements we can make (see 508e825f), but the feature
is in place and should be working correctly.
