swift234

Module interface printing: change the implementation of the underscore rule so
that important protocols are not hidden

The rule essentially remains the same -- underscored symbols are stdlib
implementation details and users should not be relying on them.  (Even
protocols like _Collection, are conceptually implementation details -- they are
not a part of the original design and we would like to remove them as soon as
the bugs are fixed.)

But since protocols define requirements for user types, we need to display even
underscored protocols, except for _Builtin* protocols, which can never be
satisfied by a user type.

rdar://16986307


Swift SVN r20083