[clang] nonblocking/nonallocating attributes (was: nolock/noalloc) (PR #84983)

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Sirraide wrote:

> Yeah, when things settle down from this latest round of feedback, I'll back out as much as possible for "phase two".

In case that helps, imo it’s mainly the stuff in `AnalysisBasedWarnings.cpp` that is kind of separate from everything else; everything else can probably just stay in this pr imo.

https://github.com/llvm/llvm-project/pull/84983