go.tools/godoc/analysis: be defensive about files without position info (such as cgo generated files)
go.tools/godoc/analysis: be defensive about files without position info (such as cgo generated files)

Also: improve log message for frontend errors without position info.

LGTM=gri
R=gri
CC=golang-codereviews
https://golang.org/cl/109100043

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