AArch64: fix weird edge case in ABI.

It turns out there are some sort-of-but-not-quite empty structs that break all
the rules. For example:

struct SuperEmpty { int arr[0]; };
struct SortOfEmpty { struct SuperEmpty e; };

Both of these have sizeof == 0, even in C++ mode, for GCC compatibility. The
first one also doesn't occupy a register when passed by value in GNU C++ mode,
unlike everything else.

On Darwin, we want to ignore the lot (and especially don't want to try to use
an i0 as we were).

llvm-svn: 302313