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

[Michael Buch via llvm-commits]

Michael137 wrote:

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

True. There are RC config files for some grep alternatives like ripgrep that we could add to llvm if this gets in people's way. Though don't think such config exists for GNU grep

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