[libcxxabi] [llvm] [ItaniumDemangle] reapply 0e754e114a6 (PR #79488)

[Nico Weber via llvm-commits]

nico wrote:

https://buildkite.com/llvm-project/libcxx-ci/builds/33217 is apparently a required check. It's been running for 3h 37m now. How about requiring required checks to complete in 20 min or so?

https://github.com/llvm/llvm-project/pull/79488