Introduce var_msym_value_operation

Introduce var_msym_value_operation

This adds class var_msym_value_operation, which implements
OP_VAR_MSYM_VALUE.  A new method is added to class operation in order
to support a special case in minsym evaluation.

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

	* expression.h (class operation) <set_outermost>: New method.
	* expop.h (class var_msym_value_operation): New.
	* eval.c (eval_op_var_msym_value): No longer static.
	(var_msym_value_operation::evaluate_for_address)
	(var_msym_value_operation::evaluate_for_sizeof)
	(var_msym_value_operation::evaluate_for_cast): New methods.
	* ax-gdb.c (var_msym_value_operation::do_generate_ax): New
	method.