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
Tim Northover