Fix parameter scoping on lambdas

[jcouv]

In working on nameof(parameter) feature (draft PR), I found a bug in scoping for lambda parameters. One-line fix.

TODO2 document breaking change

class C
{
    void M()
    {
        // error CS1503: Argument 1: cannot convert from 'int' to 'string'
        var _ = void ([My(parameter)] int parameter) => throw null;
    }
    // error CS0103: The name 'parameter' does not exist in the current context
    void M2([My(parameter)] int parameter) => throw null;
}
public class MyAttribute : System.Attribute
{
    public MyAttribute(string name1) { }
}

[jcouv]

Will merge this fix into #60098