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

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

> Is an RFC required? CC @cor3ntin

I this so, yes, this is sufficiently weird and novel that it needs an RFC - I am a bit surprised by the motivation, especially in light of MSVC successfully deploying expected with `nodiscard` - Is this a solution in search of a problem?

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