swift234

Start making @objc not start with an @ sign:
 - Change the parser to accept "objc" without an @ sign as a contextual
   keyword, including the dance to handle the general parenthesized case.
 - Update all comments to refer to "objc" instead of "@objc".
 - Update all diagnostics accordingly.
 - Update all tests that fail due to the diagnostics change.
 - Switch the stdlib to use the new syntax.

This does not switch all tests to use the new syntax, nor does it warn about
the old syntax yet.  That will be forthcoming.  Also, this needs a bit of 
refactoring, which will be coming up.



Swift SVN r19555