null default value for non-nullable parameter should warn

[jcouv]

Example

public class C 
{
    public void M(string x = null) 
    {
    }
}

For comparison, a local declaration does warn:

public class C 
{
    public void M() 
    {
        string x = null; // warning CS8600: Converting null literal or possible null value to non-nullable type.
    }
}

Found in nullable dogfood.
Tagging @cston

Note: there are some IDE tests (in CSharpDeclareAsNullableCodeFixTests) for this bug, which should be unskipped.

[jcouv]

Verified this was fixed.
sharplab