Fortran: Remove ENUM_BITFIELD in Fortran. [PR125507]

GCC requires C++14 and C++ supports enum bitfield so ENUM_BITFIELD is deprecated macro.

Bootstrapped and regtested on x86_64-pc-linux-gnu.

	PR middle-end/125507

gcc/fortran/ChangeLog:

	* gfortran.h (symbol_attribute): Remove ENUM_BITFIELD.
	(gfc_omp_namelist): Ditto.
	(gfc_omp_clauses): Ditto.
	(gfc_common_head): Ditto.
	(gfc_namespace): Ditto.
	(gfc_intrinsic_arg): Ditto.

Signed-off-by: Kael Andrew Franco <kaelfandrew@gmail.com>