Coalesce expressions do not allow throw expressions on the right hand side

[sharwell]

Version Used: 16.2 Preview 2

Steps to Reproduce:

diagnosticId = diagnosticId ?? throw new ArgumentNullException(nameof(diagnosticId));

Expected Behavior:

No suggestion is made to use ??= shorthand.

Actual Behavior:

A diagnostic appears, and the code fix changes the statement to the following invalid code:

diagnosticId ??= throw new ArgumentNullException(nameof(diagnosticId));