You have to understand that one of LLVM goal is to provide a full toolchain, through reusable library components. So we're seeking to provide our own solution for this, and we're actually providing this feature as a library for a long time. If you're on macOS, it is the default libc++-abi and you can have it available on any macOS system:

  $ nm /usr/lib/libc++abi.dylib  | grep cxa_demangle
  00000000000018e0 T ___cxa_demangle

Which means you can compile the following c++ file with clang++ and gets demangling from the system:

  #include <iostream>
  extern "C" char *__cxa_demangle(const char *mangled_name, char *buf, size_t *n, int *status);
  int main() {
    std::cout << "Demangled: " << __cxa_demangle("_Z17GetExecutablePathPKcb", nullptr, nullptr, nullptr) << "\n";
  }

We need a great demangler in LLVM because we're using it in the LLDB debugger for instance. The LLVM one is quite slow actually, if anyone is interested in writing a (clean) very performant C++ one within LLVM, I'm volunteering to help the review and integration :)

(edit: formatting)