[Issue 20982] Add a pragma to suppress deprecation messages

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20982

--- Comment #2 from Andrei Alexandrescu <andrei at erdani.com> ---
Another idea from Stefan was to control this programmaticaly:

static if (deprecated(symbol)) {
    ... avoid ...
}

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