[llvm] [Offload][OpenMP] Prettify error messages by "demangling" the kernel name (PR #101400)

[Daniel Woodworth via llvm-commits]

dwoodwor-intel wrote:

I ran into a similar issue downstream and extended `llvm::demangle` to support these function names itself. Would it make sense to do this upstream too? The format of the unmangled name would be similar to what `prettifyFunctionName` produces except without the mangled name added on the end.

https://github.com/llvm/llvm-project/pull/101400