go.tools/cmd/cover: preserve comments, as other build tools may need "// +build" directives.
go.tools/cmd/cover: preserve comments, as other build tools may need "// +build" directives.

Also: fix a crash when a file already contains 'import "sync/atomic"'.

R=r, gri
CC=golang-dev
https://golang.org/cl/14441052

glasser/golang-tools,golang/tools,josharian/tools,argandas/tools,dmitris/tools,runcom/tools,sshetty10/tools,springning/tools,nicot/tools,cmars/tools,magiccao/tools,argandas/tools,syreclabs/go-tools,nicot/tools,syreclabs/go-tools,maddyonline/tools,4ad/tools,cmars/tools,syreclabs/go-tools,maddyonline/tools,cw2018/tools,jdhenke/tools,cw2018/tools,glasser/golang-tools,4ad/tools,nicot/tools,magiccao/tools,springning/tools,dmitris/tools,matryer/tools,golang/tools,cmars/tools,runcom/tools,syreclabs/go-tools,cmars/tools,maddyonline/tools,springning/tools,sshetty10/tools,golang/tools,dmitris/tools,josharian/tools,josharian/tools,maddyonline/tools,sshetty10/tools,golang/tools,matryer/tools,matryer/tools,golang/tools,sshetty10/tools,jdhenke/tools,magiccao/tools,glasser/golang-tools,jdhenke/tools,glasser/golang-tools,dmitris/tools,cw2018/tools,jdhenke/tools,cw2018/tools,runcom/tools,argandas/tools,nicot/tools,4ad/tools,matryer/tools,syreclabs/go-tools,syreclabs/go-tools,argandas/tools,magiccao/tools,springning/tools,runcom/tools,josharian/tools