More work on translating message expressions.

Still to do:

- Chris will fix a bug in the rewriting engine for nested stmts. For example, the following:

    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init]; 

tranlates to:

    NSAutoreleasePool * pool = objc_msgSend(objc_msgSend(objc_getClass("NSAutoreleasePool"), sel_getUid("alloc")), sel_getUid("init"))utoreleasePool"), sel_getUid("alloc")) init];

...which is correct, except there is garbage after sel_getUid("init"). This is because the rewriter isn't updating the extent of the containing message expression. 

- Do the crazy cast thang.

llvm-svn: 43316