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

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Sirraide wrote:

> I would also like to hear what @Sirraide thinks

I don’t think I have any further comments; I think the implementation lgtm but I’ll leave it to @erichkeane to approve this.

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