[clang] [clang] Add the candiscard attribute to suppress nodiscard (PR #154943)

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halbi2 wrote:

> Implementing an attribute as a clang extension requires an RFC. An attribute is part of the clang frontend so as such post under that category on discourse.

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@cor3ntin you said MSVC has already deployed `[[nodiscard]] expected`, could that be used as evidence in favor?

https://github.com/llvm/llvm-project/pull/154943