Introduce var_value_operation

Introduce var_value_operation

This adds class var_value_operation, which implements OP_VAR_VALUE.

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

	* expop.h (class var_value_operation): New.
	* eval.c (var_value_operation::evaluate)
	(var_value_operation::evaluate_for_address)
	(var_value_operation::evaluate_with_coercion)
	(var_value_operation::evaluate_for_sizeof)
	(var_value_operation::evaluate_for_cast): New methods.
	* ax-gdb.c (var_value_operation::do_generate_ax): New method.