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

[Pavel Labath via llvm-commits]

labath wrote:

Fundamentally there, no reason why these tests need to use llvm symbol names. Maybe we could change (at least the most egregious offenders) to some unrelated strings (keeping the same length, to make things simple)?

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