[ffi] Support passing and returning composites (structs) by value. #36730
Description
I wanted to writing bindings for libclang (so that I can generate bindings from C/C++ header files automatically), but I discovered that libclang API uses structs passed and returned by value.
typedef struct {
unsigned int_data[4];
void *ptr_data;
} CXToken;
typedef struct {
const void *data;
unsigned private_flags;
} CXString;
CINDEX_LINKAGE CXString clang_getTokenSpelling(CXTranslationUnit, CXToken);
So I can't really bind to this API. There are some interesting design problems to resolve: e.g. when a structure is returned by value who manages the memory of that struct and how do we prevent people from making a mistake of passing address of that struct to some other method.