More changes necessary to integrate the objc and blocks rewriters.

With this commit, stuff like this is very close to working...

    [foo barf:^(int){ printf("whatever\n"); }];

Here is what is currently translates to...

    ((id (*)(id, SEL, void (^)(int)))(void *)objc_msgSend)((id)foo, sel_registerName("barf:"), (void (*)(int))__main_block_func_0);

I just need make sure the funky cast on objc_msgSend() is converted from "void (^)(int)" to "void (*)(int)". Since the cast doesn't appear in the source code, it needs to be converted in RewriteObjC::SynthMessageExpr().

llvm-svn: 58348