* data/location.cc: Sort includes.
(b4_position_file, b4_location_file): New.
When there's a file for locations but not for positions, include the
definition of position in the location file.
* data/lalr1.cc (b4_shared_declarations): Include the
position/location file when it exists.
Otherwise, define the class.
* data/glr.cc: Likewise.
* tests/input.at (%define file variables): Check them.
* tests/output.at (C++ output): Check various cases with
api.position.file and api.location.file.
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