swift234

<rdar://problem/18311362> TLF: Eliminate implicit bridging conversions

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