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

gnanderson/go,tv42/old-go,gnanderson/go,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,tv42/old-go,apparentlymart/golang,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme