[llvm] Fix mechanism propagating mangled names for TLI function mappings (PR #66656)

[Paul Walker via llvm-commits]

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

FYI: I'll take a proper look at `VecFuncs.def` once I'm happy with the rest of the patch.

https://github.com/llvm/llvm-project/pull/66656