x/tools/gopls: no type inference for builtin args in composite literals

[muirdm]

We aren't giving proper completions in this case:

var foo []int
struct { i []int }{
  i: append(nil, <>), // want "foo..."
}

I presume the composite literal is messing up the "append" param type inference.

Note that we also don't infer the type in this case:

var foo []int
append([]int{}, <>) // want "foo..."

In the first example we need to infer based on the struct field type, and in the second example we need to infer based on the first "append" param type.