Fix RCS1169

[jamesHargreaves12]

Currently, RCS1169 will suggest setting variables to read-only in cases which would cause compile-time errors such as the following:

class C
{
    int _a;
    void M2() {M(ref _a!);} 
}

This is because when working out what symbols have values assigned to them outside a constructor it currently does not consider symbols that have the SuppressNullableWarning suffix (!).

There is another minor issue is that anything inside the ref expression is not being considered by the MakeMemberReadonlyWalker.

[josefpihrt]

Nice! 👍