swift234

introduce a new 'DeclModifier' flag on attributes, which mark that the
attribute is a "modifier" of a decl, not an "attribute" and thus shouldn't
be spelt with an @ sign.  Teach the parser to parse "@foo" but reject it with
a nice diagnostic and a fixit if "foo" is a decl modifier.

Move 'dynamic' over to this (since it simplifies some code), and switch the
@optional and @required attributes to be declmodifiers (eliminating their @'s).



Swift SVN r19787