Implement unary increment and decrement operations

Implement unary increment and decrement operations

This implements the unary increment and decrement operations, "++" and
"--".

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

	* expop.h (unop_incr_operation): New template.
	(preinc_operation, predec_operation, postinc_operation)
	(postdec_operation): New typedefs.
	* eval.c (eval_op_preinc, eval_op_predec, eval_op_postinc)
	(eval_op_postdec): No longer static.