Implement some unary operations

Implement some unary operations

This introduces a couple of new template classes and then uses them to
implement some simple unary operations.

gdb/ChangeLog
2021-03-08  Tom Tromey  <tom@tromey.com>

	* expop.h (unary_ftype): New typedef.
	(unop_operation, usual_ax_binop_operation): New templates.
	(unary_plus_operation, unary_neg_operation)
	(unary_complement_operation, unary_logical_not_operation): New
	typedefs.
	* eval.c (eval_op_plus, eval_op_neg, eval_op_complement)
	(eval_op_lognot): No longer static.
	* ax-gdb.c (gen_expr_unop): New function.