message: [SwiftASTContext] Improve an error message.
  No need to show the users so many details. Discussed with
  Jim/Adrian.
diff: @@ -3876,8 +3876,7 @@ SwiftASTContext::GetTypeFromMangledTypename(const char *mangled_typename,
  if (!mangled_typename ||
  !SwiftLanguageRuntime::IsSwiftMangledName(mangled_typename)) {
  error.SetErrorStringWithFormat(
  - "typename '%s' is not a valid Swift mangled "
  - "typename, it should begin with " MANGLING_PREFIX_STR,
  + "typename '%s' is not a valid Swift mangled name",
  mangled_typename);
  return CompilerType();
  }
  