spec: fallthrough may not appear in last clause of a switch
spec: fallthrough may not appear in last clause of a switch

Replacement for CL 7370053 which attempted to make fallthrough's
syntactically a part of switch statements. Because they may be
labeled, fixing that CL completely would require too much spec
surgery.

Fixes issue 4923.

R=r, iant, rsc, ken
CC=golang-dev
https://codereview.appspot.com/7416048

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