Pull SE-0081 into Swift 3.0 Preview 1 (#2850)
* Implement parser support for SE-0081 - Move 'where' clause to end of declaration
This patch includes testsuite changes to show each of the decls supported.
Next step is to migrate the stdlib + testsuite + corelibs: I'd would *greatly* appreciate help with this.
After that is done, deprecation + migration of the old form can happen.
* My recent patch "broke" these tests, but allowing the syntax in them to trigger existing generics bugs.
