code coverage: broken when file contains //line directives

[pjweinb]

In the latest version of vscode-go, and probably in all versions, code containing //line directives does not show coverage highlighting.

A short program with first lines

//
package main

produces a coverage file:

mode: set
foo.gak/main.go:6.10,8.26 2 1
foo.gak/main.go:12.2,12.19 1 1
foo.gak/main.go:8.26,11.3 2 1
foo.gak/main.go:12.19,13.21 1 0

If the first line is changed to
//line con.go:8
the coverage file changes to:

mode: set
foo.gak/main.go:12.0,14.0 2 1
foo.gak/main.go:18.0,18.0 1 1
foo.gak/main.go:14.0,17.0 2 1
foo.gak/main.go:18.0,19.0 1 0

which displays nothing