Fix an inconsistent state, between the in_common attribute
and the common_block pointer.

 - adding a symbol to a common block list in gfc_match_common is delayed
   after the call to gfc_add_in_common.
 - gfc_restore_latest_undo_checkpoint is changed to check the common_block
   pointer directly instead of the in_common attribute.
 - gfc_restore_old_symbol is changed to also restore
   the common-related pointers.  This is done using a new function created
   to factor the related memory management.
 - In gfc_restore_last_undo_checkpoint, when a symbol has been removed
   from the common block linked list, its common_next pointer is cleared.

	PR fortran/67758
gcc/fortran/
	* gfortran.h (gfc_symbol): Expand comment.
	* match.c (gfc_match_common): Delay adding the symbol to
	the common_block after the gfc_add_in_common call.
	* symbol.c (gfc_free_symbol): Move common block memory handling...
	(gfc_set_symbol_common_block): ... here as a new function.
	(restore_old_symbol): Restore common block fields.
	(gfc_restore_last_undo_checkpoint):
	Check the common_block pointer instead of the in_common attribute.
	When a symbol has been removed from the common block linked list,
	clear its common_next pointer.
gcc/testsuite/
	* gfortran.dg/common_25.f90: New file.

From-SVN: r228947