[Clang][Sema] Add Fortify Warnings for more functions

[Sharjeel-Khan]

Android has local fortify warnings for functions not seen in clang. My plan is to upstream them over time. This issue lists all the functions and will track the progress on upstreaming them:

• poll.h
  • poll
  • ppoll/ppoll64
• socket.h
  • recvfrom
  • sendto
  • recv
  • send
• stdio.h
  • fread
  • fwrite
  • fgets
• string.h/strings.h
  • stpcpy
  • strcat
  • strlcpy
  • strlcat
  • bcopy
  • bzero
• unistd.h
  • getcwd
  • pread/pread64
  • pwrite/pwrite64
  • read
  • readlink
  • readlinkat
  • write