re PR libfortran/91593 (Implicit enum conversions in libgfortran/io/transfer.c)

2019-09-28  Jerry DeLisle  <jvdelisle@gcc.gnu.org>

	PR libfortran/91593
	* io/io.h: Add gcc_unreachable().
	* io/transfer.c (file_mode, current_mode,
	formatted_transfer_scalar_read, formatted_transfer_scalar_write,
	pre_position, next_record_r, next_record_w): Add and use
	FORMATTED_UNSPECIFIED to enumeration.

From-SVN: r276255