[libcxxabi] [llvm] [ItaniumDemangle][test] Factor demangler test-cases into file and sync into LLVM (PR #137947)

[Nikita Popov via llvm-commits]

nikic wrote:

An annoying consequence of this change is that grepping in `llvm/` now produce a lot of noise by matching these test cases.

https://github.com/llvm/llvm-project/pull/137947