[llvm] [VFABI] Refactor try demangle for vfabi to use only the vector abi mangled names (PR #67430)

[Paul Walker via llvm-commits]

https://github.com/paulwalker-arm commented:

Given my current comments I've not reviewed the `tools` and `unittests` changes but broadly this is looking like a nice cleanup, you just need to firm up which type is used to calculate the ElemetCount.

https://github.com/llvm/llvm-project/pull/67430