Translate socket functions in C mode

[lucic71]

getsockname, and the rest of the socket functions that receive a struct sockaddr* (__SOCKADDR_ARG) parameter, are defined differently in C and C++:

int getsockname (int fd, __SOCKADDR_ARG addr, socklen_t *len)

#if defined __cplusplus
  #define __SOCKADDR_ARG		struct sockaddr *__restrict
#else
  typedef union {
    struct sockaddr *__restrict __sockaddr__;
    struct sockaddr_in *__restrict __sockaddr_in__;
  } __SOCKADDR_ARG __attribute__ ((__transparent_union__));
#endif

__transparent_union__ is an attribute that allows a function whose parameter has that union type can be called with any of the union's arm types directly, without a cast:

struct sockaddr_in in;
// this would normally generate an error because &in has type
// sockaddr_in* but the argument is declared sockaddr*
getsockname(fd, &in, len);

struct sockaddr_in6 in6;
getsockname(fd, &in6, len); // same tension between sockaddr_in6* and sockaddr_in*

__transparent_union is only available in C. In C++, an explicit cast is required.

In the Clang AST, the transparent union object is created through a CompoundLiteralExpression(InitListExpr(struct sockaddr_in *)). We convert the argument of CompoundLiteralExpression in order to get the correct rust code.