Define unconstify() and unvolatize() for C++.

Define unconstify() and unvolatize() for C++.

These two macros wouldn't work if used in an inline function definition
in a header seen by g++, because __builtin_types_compatible_p is only
available in C.  Redirect to standard C++ const_cast (which also
adds/removes volatile despite its name).

Per cpluspluscheck failure in a development branch.

Suggested-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/CA%2BhUKGK3OXFjkOyZiw-DgL2bUqk9by1uGuCnViJX786W%2BfyDSw%40mail.gmail.com