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

[Doug Wyatt via cfe-commits]

dougsonos wrote:

> One thing I noticed you bring up on Discourse and which I agree is probably for the better: this pr can just be about introducing an effect system; actually using it can be deferred to a follow-up pr. This pr is already big enough if you consider just those changes, and there’s already enough to consider here to the point that _also_ trying to review the analysis part of this here would probably get too messy...

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

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