cmd/cover: use different //line comment for code that does not come from source file

What version of Go are you using (go version)?

go version go1.10.1 darwin/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

GOARCH="amd64" GOEXE="" GOHOSTARCH="amd64" GOHOSTOS="darwin" GOOS="darwin" GORACE="" GOTMPDIR="" GCCGO="gccgo" CC="clang" CXX="clang++" CGO_ENABLED="1" CGO_CFLAGS="-g -O2" CGO_CPPFLAGS="" CGO_CXXFLAGS="-g -O2" CGO_FFLAGS="-g -O2" CGO_LDFLAGS="-g -O2" PKG_CONFIG="pkg-config" GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/m6/vs051rl54k3_sgxr94ylcz5h0000gn/T/go-build238749509=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

git clone git@github.com:linzhp/go_examples.git
cd go_examples/hyphen
go tool cover -mode set -var Var_power-driven -o power-driven-gen.go power-driven.go
go run main.go

What did you expect to see?

An error pointing to the right line of power-driven-gen.go

What did you see instead?

power-driven.go:7:14: expected ';', found '-'

However, power-driven.go has less than 7 lines.