swift234

Weak-link to Objective-C classes that Clang considers to be weak-imported.

This is a simple, trivialy, not-even-half-way-there solution to weak
leaking of Objective-C classes introduced after the deployment
target. It only works for Objective-C classes and C global variables
that Clang consideres to be "weak imported". However, this bare
minimum should be enough to develop an app (by jumping through various
hoops) that uses new functionality when its
available. <rdar://problem/17296490>, which I've restricted in scope
to capture this.

Swift SVN r20956