[clang] nonblocking/nonallocating attributes: 2nd pass caller/callee analysis (PR #99656)

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Sirraide wrote:

> I feel like this might be the way to go: have some attribute that makes the analyser just ignore calls to a function entirely

Actually, I just saw that you settled on `[[noreturn]]` + `noexcept`, which seems like a reasonable combination. I don’t think we need an extra attribute in that case, but what we should do is document this behaviour somewhere (actually, do we already have docs for effects? if not, we definitely should; our documentation is generally not that great, so it would be nice to improve that).

https://github.com/llvm/llvm-project/pull/99656