libgdiagnostics: add accessors for diagnostic_logical_location [LIBGDIAGNOSTICS_ABI_1]

libgdiagnostics: add accessors for diagnostic_logical_location [LIBGDIAGNOSTICS_ABI_1]

For followup work I need to be able to get at data from a
diagnostic_logical_location after creating it, hence the
need to extend libgdiagnostics with accessor entrypoints.

This is the first extension to libgdiagnostics since the initial
release.  The patch uses symbol versioning to add the new
entrypoints in the same way that libgccjit does.

gcc/ChangeLog:
	* doc/libgdiagnostics/topics/compatibility.rst: New file, based
	on gcc/jit/docs/topics/compatibility.rst.
	* doc/libgdiagnostics/topics/index.rst: Add compatibility.rst.
	* doc/libgdiagnostics/topics/logical-locations.rst (Accessors):
	New section.
	* libgdiagnostics++.h (logical_location::operator bool): New.
	(logical_location::operator==): New.
	(logical_location::operator!=): New.
	(logical_location::get_kind): New.
	(logical_location::get_parent): New.
	(logical_location::get_short_name): New.
	(logical_location::get_fully_qualified_name): New.
	(logical_location::get_decorated_name): New.
	* libgdiagnostics.cc
	(diagnostic_logical_location::get_fully_qualified_name): New.
	(diagnostic_logical_location_get_kind): New entrypoint.
	(diagnostic_logical_location_get_parent): New entrypoint.
	(diagnostic_logical_location_get_short_name): New entrypoint.
	(diagnostic_logical_location_get_fully_qualified_name): New
	entrypoint.
	(diagnostic_logical_location_get_decorated_name): New entrypoint.
	* libgdiagnostics.h
	(LIBDIAGNOSTICS_HAVE_LOGICAL_LOCATION_ACCESSORS): New define.
	(diagnostic_logical_location_get_kind): New entrypoint.
	(diagnostic_logical_location_get_parent): New entrypoint.
	(diagnostic_logical_location_get_short_name): New entrypoint.
	(diagnostic_logical_location_get_fully_qualified_name): New
	entrypoint.
	(diagnostic_logical_location_get_decorated_name): New entrypoint.
	* libgdiagnostics.map (LIBGDIAGNOSTICS_ABI_1): New.

gcc/testsuite/ChangeLog:
	* libgdiagnostics.dg/test-logical-location.c: Include
	<string.h>.
	(main): Verify that the accessors work.
	* libgdiagnostics.dg/test-logical-location.cc: New test.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>