Implement UNOP_MEMVAL and UNOP_MEMVAL_TYPE

Implement UNOP_MEMVAL and UNOP_MEMVAL_TYPE

This adds class unop_memval_operation and unop_memval_type_operation,
which implement UNOP_MEMVAL and UNOP_MEMVAL_TYPE.

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

	* expop.h (class unop_memval_operation)
	(class unop_memval_type_operation): New.
	* eval.c (eval_op_memval): No longer static.
	(unop_memval_operation::evaluate_for_address)
	(unop_memval_type_operation::evaluate_for_address)
	(unop_memval_operation::evaluate_for_sizeof)
	(unop_memval_type_operation::evaluate_for_sizeof): New methods.
	* ax-gdb.c (unop_memval_operation::do_generate_ax)
	(unop_memval_type_operation::do_generate_ax): New methods.