When compiling with C++ interop enabled, we enable extra safety checks to prevent library authors from accidentally exposing ABI-fragile C++ symbols in resilient Swift interfaces.
The heuristic we use is overly strict, and it prevents the compiler from being able to typecheck various modules from their interfaces when C++ interop is enabled. Darwin and System are two of such modules.
The underlying challenge is that there isn't a good distinction between C structs and C++ structs: whenever parsing a header file in C++ language mode, Clang assumes that every struct is a C++ struct.
This relaxes the heuristic to allow exposing C-like structs in resilient interfaces.
rdar://140203932
