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

[via cfe-commits]

Sirraide 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...

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