objc IRGen for Next runtime message API.

objc IRGen for Next runtime message API.
The prototype for objc_msgSend() is technically variadic -
`id objc_msgSend(id, SEL, ...)`.
But all method calls should use a prototype that matches the method,
not the prototype for objc_msgSend itself().
// rdar://9048030

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@126754 91177308-0d34-0410-b5e6-96231b3b80d8