x/tools/gopls: gopls is confused by //line directives

[pjweinb]

go1.15.1 darwin/amd64

Files with //line directives confuse gopls, for instance no hover returned, and no diagnostics generated.

In

//line a.y:1
package main

func main() {
	var err error
	err.Error()
}

hovering on Error returns nothing, but it hovers fine without the //line comment.

Likewise, in (the -- <name> -- headers say what the contents of are)

-- a.go --
package consts

const a = 1
-- b.go --
package consts
//line gen.go:5
const a = 2

no errors are generated for b.go as it stands (although the definition in a.go is described as the 'other' definition), while errors are generated correctly without the //line comment.

[pjweinb]

also noted in #33690