Require 'as' when converting from Objective-C type to native type (but
continue to allow implicit conversion from native to Objective-C). This
conversion constraint is called ExplicitConversion; all implicit
conversions are covered by the existing Conversion constraint. Update
standard library and tests to match.
Swift SVN r24496
in r19361. Thank you Doug for pointing out that I did
not import Foundation. Sigh, that build systems have different
behavior in buildbot and elsewhere. Hope this fixes the buildbot failure.
// rdar://16726822
Swift SVN r19367
