Records: Disallow ref kinds in positional members

[jcouv]

We should disallow ref kinds: record C(ref int i);.
Not sure what we want to do with params: record C(params int[] i)

We should also clarify what we want to do for default values:

record C(int X = 42) // should probably be an error, since we're silently dropping the default value 42
{
   public int X { get; init; } = 43;
}

Maybe we do: "no parameter default values when the property isn't synthesized" or maybe we just disallow default values for now.

[RikkiGibson]

ref/out/in mods clearly need to be disallowed, but I feel like there are probably reasonable use cases for params [] and for default values when the property isn't synthesized.