cmd/cc: reject unions containing pointers
cmd/cc: reject unions containing pointers

If a union contains a pointer, it will mess up the garbage collector, causing memory corruption.

R=golang-dev, dave, nightlyone, adg, dvyukov, bradfitz, minux.ma, r, iant
CC=golang-dev
https://codereview.appspot.com/8469043

d0f/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,bryanxu/go-zh,mhennings/marcohennings-go,mhennings/marcohennings-go,d0f/go-zh,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,webfd/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,webfd/go-zh,d0f/go-zh,mhennings/marcohennings-go,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,webfd/go-zh,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,webfd/go-zh,webfd/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,bryanxu/go-zh,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,webfd/go-zh,mhennings/marcohennings-go,mhennings/marcohennings-go,mhennings/marcohennings-go,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,bryanxu/go-zh,rdp/rogerpack2005-golang