Suggest pattern names for union case fields

[kerams]

Implements #1885.

Names are suggested in match clause, binding and lambda patterns of arbitrary nesting. Functions and values also no longer clutter the completions, since they are not legal in patterns.

The initial high priority of these suggested names does get lost as you start typing though, because VS sorts the list on its own.

[auduchinok]

Do the following cases still work with this change?

Some, None, Nested, U, A, Id, Num should all still be available:

module Nested =
    type U =
        | A

    let (|Id|) x = x

    let [<Literal>] Num = 1

open Nested

match None with
| Some({caret})

The same, but without the parens:

match None with
| Some {caret}

[kerams]

All of those items are present.

[auduchinok]

Functions and values also no longer clutter the completions, since they are not legal in patterns.

All of those items are present.

It's interesting, given that Num is a value. 🙂