[Bug c/71255] Implement #pragma may_alias

[fw at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71255

--- Comment #14 from Florian Weimer <fw at gcc dot gnu.org> ---
I believe the equivalent C test case:

#include <stdlib.h>

struct sockaddr;
struct sockaddr *f();

struct __attribute__((may_alias)) sockaddr {};
struct sockaddr *f()
{
  return NULL;
}

still does not work:

t.c:7:18: error: conflicting types for ‘f’
 struct sockaddr *f()
                  ^
t.c:4:18: note: previous declaration of ‘f’ was here
 struct sockaddr *f();