Improve the "not exhaustive" diagnostic in the presence of a when clause.

[gafter]

After #44702 we produce a diagnostic that gives an example of a pattern that would cover a non-covered scenario in a switch expression that isn't exhaustive. However, we don't tell the user when that is because of the presence of a when clause. @alrz observed this in code review:

I wonder what example we give for:

return unsigned switch 
{
  0 => true,
  var x when x > 0 => false,
};

I would expect _ since any other number is caught by the second arm. F# gives the example anyways but has a special diagnostic in presence of when. For example, the value '1' may indicate a case not covered by the pattern(s). However, a pattern rule with a 'when' clause might successfully match this value.

We should definitely enhance the diagnostic to include this information when appropriate.