Change constraint mismatch for 'supports null' from warning to error

[T-Gro]

With the nullness feature being added, there is a new constraint solving codepath for nullness-related mismatches.
All nullness issues were warnings by default.

However, together with the 'supports null' constraint (e.g. matching on null, assigning null), this could lead to unsound F# code, e.g. by assigning null to an int.

This changes the logic by keeping it an error if an unsound type is inferred to have the 'null' constraint (coming from generics, assignment, matching).

Fixes #17423

let test = 
    match null with
    | null -> true
    | 42 -> false

[github-actions]

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