Build LLVM demangler directly with our sources

This remove dep on LLVM library by building the demangler as part of the
project.

This should help with building on macOS.

external/llvm/Demangle/Demangle.cpp

@@ -0,0 +1,36 @@
+//===-- Demangle.cpp - Common demangling functions ------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file This file contains definitions of common demangling functions.
+///
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Demangle/Demangle.h"
+#include <cstdlib>
+
+static bool isItaniumEncoding(const std::string &MangledName) {
+  size_t Pos = MangledName.find_first_not_of('_');
+  // A valid Itanium encoding requires 1-4 leading underscores, followed by 'Z'.
+  return Pos > 0 && Pos <= 4 && MangledName[Pos] == 'Z';
+}
+
+std::string llvm::demangle(const std::string &MangledName) {
+  char *Demangled;
+  if (isItaniumEncoding(MangledName))
+    Demangled = itaniumDemangle(MangledName.c_str(), nullptr, nullptr, nullptr);
+  else
+    Demangled = microsoftDemangle(MangledName.c_str(), nullptr, nullptr,
+                                  nullptr, nullptr);
+
+  if (!Demangled)
+    return MangledName;
+
+  std::string Ret = Demangled;
+  free(Demangled);
+  return Ret;
+}