[ObjC++] Make parameter passing and function return compatible with ObjC

[ObjC++] Make parameter passing and function return compatible with ObjC

ObjC and ObjC++ pass non-trivial structs in a way that is incompatible
with each other. For example:

typedef struct {
  id f0;
  __weak id f1;
} S;

// this code is compiled in c++.
extern "C" {
  void foo(S s);
}

void caller() {
  // the caller passes the parameter indirectly and destructs it.
  foo(S());
}

// this function is compiled in c.
// 'a' is passed directly and is destructed in the callee.
void foo(S a) {
}

This patch fixes the incompatibility by passing and returning structs
with __strong or weak fields using the C ABI in C++ mode. __strong and
__weak fields in a struct do not cause the struct to be destructed in
the caller and __strong fields do not cause the struct to be passed
indirectly.

Also, this patch fixes the microsoft ABI bug mentioned here:

https://reviews.llvm.org/D41039?id=128767#inline-364710

rdar://problem/38887866

Differential Revision: https://reviews.llvm.org/D44908

llvm-svn: 328731