"GCC emits an __objc_class_name_{classname} symbol for every class, and a...

"GCC emits an __objc_class_name_{classname} symbol for every class, and a corresponding reference to this symbol for every compilation unit that references the class.  This causes linker errors when you try linking a program which references some classes but doesn't define them.  The attached patch implements this support in clang, so you can compile a class with clang, reference it in a file compiled with GCC, and have it all work correctly."

Patch by David Chisnall!

llvm-svn: 73364