Allow eliding qualification when pattern matching against DUs where type is known

I propose we allow eliding module/type qualifications when pattern matching against DUs where the type is known.

Today, for records, F# is smart enough to not require qualifying the type when it knows which type you are matching against:

```f#
type A = { X: int }

type B = { X: int }

let f () = { A.X = 1 }

match f () with
| { X = x } -> x
```

However, this is not the case for DUs. From a dev perspective, I see no reason why I shouldn't be able to do this:

```f#
type A = | Case1

type B = | Case1

let f () = A.Case1

match f () with
| Case1 -> ()
```

However, this fails on `Case1` on the last line because it thinks I am pattern matching against `B`:


So currently, we must qualify the case:

```f#
match f () with
| A.Case1 -> ()
```

With longer type names nested inside modules, like `SelfAttestedCourseCompletion.CreateError`, the pattern matching starts to get fairly verbose.

### Interop with active patterns

If there is an active pattern in scope with the same name, that should of course still take precedence.

## Pros and Cons

**The advantages of making this adjustment to F# are:** Consistency with record features, less verbosity without loss of clarity.

**The disadvantages of making this adjustment to F# are:** None?

## Extra information

**Estimated cost (XS, S, M, L, XL, XXL):** Don't know, maybe S-M?

**Related suggestions:** (put links to related suggestions here)

## Affidavit (please submit!)

Please tick these items by placing a cross in the box:
* [x] This is not a question (e.g. like one you might ask on [StackOverflow](http://stackoverflow.com)) and I have searched StackOverflow for discussions of this issue
* [x] This is a language change and not purely a tooling change (e.g. compiler bug, editor support, warning/error messages, new warning, non-breaking optimisation) belonging to [the compiler and tooling repository](https://github.com/dotnet/fsharp)
* [x] This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it
* [x] I have [searched both open and closed suggestions on this site](http://github.com/fsharp/fslang-suggestions/issues) and believe this is not a duplicate

Please tick all that apply:
* [x] This is not a breaking change to the F# language design
* [ ] I or my company would be willing to help implement and/or test this

## For Readers

If you would like to see this issue implemented, please click the :+1: emoji on this issue. These counts are used to generally order the suggestions by engagement.

No implementation files, tests, or compiler entry points are named. Start by reproducing the record and discriminated-union examples, then investigate F# name resolution for patterns, including precedence for same-named active patterns; done means unqualified DU cases work when the matched type is known without changing active-pattern precedence.

Written by the indexing model from the issue text.