At some point I want to propose a revised model for exports, but for now
just mark that support for '@exported' is still experimental and subject
to change. (Thanks, Max.)
In most cases this means adding @public to things that get serialized;
in a few cases it means using a modern public stdlib API instead of
a legacy thing I was trying to keep @internal.
Swift SVN r19350
Instead of referencing them by name, serialize a reference to their
storage decl instead, then note which accessor to retrieve.
Uncovered by Chris's override work in r15051...thanks, Chris!
Swift SVN r15063
