[Proposal]: `using` with a discard

[333fred]

using with a discard

• Discussion: https://github.com/dotnet/csharplang/discussions/8605

Summary

Permit using variable declarations with a discard variable:

void M()
{
    using _ = GetDisposable();
}

Design meetings

• https://github.com/dotnet/csharplang/blob/master/meetings/2018/LDM-2018-12-03.md#discussion