gdb/fortran: Add 'LOC' intrinsic support.

gdb/fortran: Add 'LOC' intrinsic support.

LOC(X) returns the address of X as an integer:
https://gcc.gnu.org/onlinedocs/gfortran/LOC.html

Before:
(gdb) p LOC(r)
No symbol "LOC" in current context.

After:
(gdb) p LOC(r)
$1 = 0xffffdf48

gdb/ChangeLog:
2021-03-09  Felix Willgerodt  <felix.willgerodt@intel.com>

        * f-exp.h (eval_op_f_loc): Declare.
        (expr::fortran_loc_operation): New typedef.
        * f-exp.y (exp): Handle UNOP_FORTRAN_LOC after parsing an
        UNOP_INTRINSIC.
        (f77_keywords): Add LOC keyword.
        * f-lang.c (eval_op_f_loc): New function.
        * std-operator.def (UNOP_FORTRAN_LOC): New operator.

gdb/testsuite/ChangeLog:
2020-03-09  Felix Willgerodt  <felix.willgerodt@intel.com>

        * gdb.fortran/intrinsics.exp: Add LOC tests.