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.

doc/bison.texinfo

@@ -3800,7 +3800,7 @@ For instance, if your locations use a file name, you may use
 %parse-param @{ char const *file_name @};
 %initial-action
 @{
-  @@$.begin.filename = @@$.end.filename = file_name;
+  @@$.initialize (file_name);
 @};
 @end example