Consider renaming `Symbol` to be less likely to collide and to be consistent with the renaming of its derived types.

```C#
    // There aren't any "Symbol" classes in the shared runtime, but this type name existing feels like it'll be a big conflict for
    // Roslyn, or any other sort of compiler/lexer/etc.
    // Is there a two-or-more-particle name that would be almost as good?
    public abstract class Symbol : ICompletionSource 
    {
```

Related: #1892