add compile-time warning about ignoring returned StatusCode
as [[nodiscard]] is C++17, use [[gnu:warn_unused_result]], which works
on both gcc and clang -- see https://godbolt.org/g/RsFcwj -- instead.
Edited by Marco Clemencic
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as [[nodiscard]] is C++17, use [[gnu:warn_unused_result]], which works
on both gcc and clang -- see https://godbolt.org/g/RsFcwj -- instead.