swift234

IRGen: Fix OBJC_CLASS_$_ clash if Clang emits first

IRGen emits lazily-emitted definitions after emitting Clang declarations.
So an internal function that references an Objective-C class global and
is itself referenced from after a public function can observe
a OBJC_CLASS_$_ global defined by Clang.

Use a similar strategy as the Clang patch, just bitcast the global if it
exists already.

Fixes <rdar://problem/23200656>.

Swift SVN r32845