[Proposal]: Show obsolete errors from overrides

[333fred]

Show obsolete errors from overrides

• Specification: None yet.
• Discussion: Obsoletion warning is not shown for obsolete overrides of non-obsolete base members #8937

Summary

Today, when an override of a virtual member adds Obsolete, we do not warn that the method is obsoleted when the derived type is statically known as the receiver. For example:

abstract class Base
{
    public abstract void M();
}

class Derived : Base
{
    [Obsolete]
    public override void M() => throw null;

    public void M2()
    {
        M(); // No warning
    }
}

This is something that we can detect at warn about.

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