go.tools/go/loader: Add Program.FilePath convenience method for getting the full path of a source file.
go.tools/go/loader: Add Program.FilePath convenience method for getting the full path of a source file.

LGTM=gri
R=gri, adonovan
CC=golang-codereviews
https://golang.org/cl/107160049

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