cmd/cc: restore correct meaning of %Q

g% 6c ~/x.c
/Users/rsc/x.c:1 duplicate types given: STRUCT s and VOID
/Users/rsc/x.c:1 no return at end of function: f
g%

Fixes issue 6083.

R=ken2
CC=golang-dev
https://codereview.appspot.com/12691043