[libcxxabi] [llvm] [ItaniumDemangle] Unconditionally parse substitution template arguments (PR #131970)

[Louis Dionne via llvm-commits]

https://github.com/ldionne commented:

It looks like we might be able to make additional simplifications to the code since I think all meaningful references to `TryToParseTemplateArgs` are being removed in this patch?

https://github.com/llvm/llvm-project/pull/131970