internal/lsp/source: improve type switch case completion

Now we will filter out the types already used in other case
statements:

    switch ast.Node(nil).(type) {
    case *ast.Ident:
    case *ast.I<> // don't offer "Ident" since it has been used
    }

Note that the implementation was not able to use a map to track the
seen types.Types because we build up types.Type entries dynamically
when searching for completions (e.g. types.NewPointer() to make a
pointer type). We must use types.Identical() instead of direct pointer
equality.

Change-Id: I316638bb48bfd6802e2caea671f297d640291010
Reviewed-on: https://go-review.googlesource.com/c/tools/+/247098
Run-TryBot: Muir Manders <muir@mnd.rs>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>