gdb/fortran: add support for RANK keyword

gdb/fortran: add support for RANK keyword

gfortran supports the RANK keyword, see:

  https://gcc.gnu.org/onlinedocs/gfortran/RANK.html#RANK

this commit adds support for this keyword to GDB's Fortran expression
parser.

gdb/ChangeLog:

	* f-exp.h (eval_op_f_rank): Declare.
	(expr::fortran_rank_operation): New typedef.
	* f-exp.y (exp): Handle UNOP_FORTRAN_RANK after parsing an
	UNOP_INTRINSIC.
	(f77_keywords): Add "rank" keyword.
	* f-lang.c (eval_op_f_rank): New function.
	* std-operator.def (UNOP_FORTRAN_RANK): New operator.

gdb/testsuite/ChangeLog:

	* gdb.fortran/rank.exp: New file.
	* gdb.fortran/rank.f90: New file.