Declare as nullable code fix for CS8600 breaks when casts are involved

[jroessel]

Product and Version Used:

Steps to Reproduce:

The following code reports CS8600 because we cast a nullable type to a non-nullable type as part of a variable declaration.

    private object? Get() => null;

    internal void Test() {
      var s = (string) Get(); // (1)
      string s2 = (string) Get(); // (2)
    }

Roslynator's code fix for CS8600 erroneously assumes the declaration type to be the problem, when it's actually the cast. So we don't get rid of the warning and in case of var actually introduce broken code.

Actual Behavior:

      var? s = (string) Get(); // (1)
      string? s2 = (string) Get(); // (2)

Expected Behavior:

      var s = (string?) Get(); // (1)
      string? s2 = (string?) Get(); // (2)

(Although in case of (2) the declaration could be adjusted by another code fix for the same diagnostic, so at least iteratively applying fixes will arrive at the same result.)

[jroessel]

Currently attempting a fix.