Let position and location be PODs.

Let position and location be PODs.
* data/location.cc (position::initialize, location::initialize): New.
(position::position, location::location): Define only if
b4_location_constructors is defined.
* data/lalr1.cc (b4_location_constructors): Define it for backward
compatibility.
* doc/bison.texinfo (Initial Action Decl): Use initialize.