Add debugging step points for "when" clauses in pattern matching

[dsyme]

At the moment we don't lay down any debug points for "when" clauses in pattern matching, so you can't break on them unless there is some sequential logic inside (e.g. let, sequencing, loops etc.)

There is no reason for this - it is simple enough to allow debug points here.

e.g.

let test1 x y =
    match x with
    | [_] when y > 4 -> 5
    | [_] when y < 4 -> -5
    | _ -> 2

Check can you validate a breakpoint on the when (before debug launch), and can you hit it at runtime.

[dsyme]

This is implemented in #11981 though there is an apparent glitch with union patterns

    let TestMatchWithWhen x  =
        match x with
        | [_;x] 
        | [x] when x < 4 -> -5  // The debugpoint for the 'when' is not hit for a two-element list
        | _ -> 2


    TestMatchWithWhen [4;5]
    TestMatchWithWhen [5;4]
    TestMatchWithWhen [6]

[dsyme]

Fixed in #11981