x/tools/gopls: do not format broken function call site

[hyangah]

Save this code:

package pkg

import (
	"net/http"
)

func handler(w http.ResponseWriter, r *http.Request) {
	http.Error(w, ) // incomplete
}

See the "," in http.Error(w, ) // incomplete is removed.

I think we shouldn't format the line with syntax errors.

[muirdm]

I don't think the trailing comma is a syntax error. To do this we would need to look at the type checker error about missing argument, but that would be a significant departure from gofmt.

[adonovan]

As Muir explains, formatting is a very shallow operation that works on any valid parse tree. It does not and cannot use types as part of its formatting decisions.

[hyangah]

I hoped we could utilize gopls's internal state before handling the format LSP request - so make formatting noop. However, I understand some users prefer formatting code regardless of the presence of type errors.

This case makes me more convinced vscode-go's default behavior that "formats on save" by default is not the right choice. (golang/vscode-go#1815)