Tracking suggestion: Active patterns disables the detection of simple duplicates cases.

Tracking this suggestion: https://github.com/Microsoft/visualfsharp/issues/2754

Presence of active patterns disables the detection of simple duplicates cases.
```fsharp
type Foo = A | B | C

let good = function
| A -> ()
| B -> ()
| B -> () // Squiggly with warning "This rule will never be matched"
| _ -> ()
| _ -> () // Squiggly with warning "This rule will never be matched"

let (|IsA|_|) x = if x = A then Some () else None

let bad = function
| IsA -> ()
| B -> ()
| B -> () // no warning
| _ -> ()
| _ -> () // no warning
```
When active patterns are present checking whether a match is exhaustive clearly becomes difficult/impossible but duplicate cases can still be detected.