swift234

- Introduce a new 'nonmutating' context sensitive keyword, and use it instead of @!mutating.
- Change the parser to unconditionally reject @mutating and @!mutating with a fixit and 
  specific diagnostic to rewrite them into the [non]mutating keyword.
- Update tests.

This resolves <rdar://problem/16735619> introduce nonmutating CS keyword and remove the attribute form of mutating all together



Swift SVN r16892