[Version 13.0] Feature support for Lock objects

[RexJaeschke]

This is Rex's adaptation of the corresponding MS proposal.

[KalleOlaviNiemitalo]

A loophole occurs to me:

class C {
    void M() {
        var l = new System.Threading.Lock();
        lock (Launder(l)) {
        }
    }
    static object Launder<T>(T v) => v;
}

But that seems unlikely to happen by accident, and anyone doing it deliberately could just disable the warning instead.