x/tools/gopls: improve completions with mutli-return value functions

[muirdm]

We should make completion handle functions that return multiple values. For example:

// uprank functions that return two values
f, err := <>

var f *os.File
var err error

// uprank functions that return (*os.File, error)
f, err = <>

myFunc := func(int, bool) {}

// uprank functions that return (int, bool)
myFunc(<>)

All cases need to continue offering the existing scalar completions.

[gopherbot]

Thank you for filing a gopls issue! Please take a look at the Troubleshooting guide, and make sure that you have provided all of the relevant information here.

[gopherbot]

Change https://golang.org/cl/215538 mentions this issue: internal/lsp/source: handle completing into variadic params

[gopherbot]

Change https://golang.org/cl/215537 mentions this issue: internal/lsp/source: improve completion involving multiple return values