Two additions...

- Synthesize the funky cast for objc_msgSend(). For the basic case, it looks like...

((id (*)(id, SEL))(void *)objc_msgSend)(obj, sel);

The "void *" cast is needed to workaround a GCC "bandaid" (Chris says it has something to do with the inliner). Without the extra "void *" cast, we get spurious warnings/notes that look like...

xx.m:17: warning: function called through a non-compatible type
xx.m:17: note: if this code is reached, the program will abort

- Add prototypes for the ObjC functions we call, objc_msgSend/objc_getClass for now (don't depend on them being included).

llvm-svn: 43685