Implement function call operations

Implement function call operations

This implement function call operations.

The current function call code relies on some very lengthy code
(evaluate_funcall is 398 lines...) to distinguish between the
different opcodes that might appear in the callee position.

Rather than try to replicate this, and have a function that tried to
dissect many different kinds of operation subclass, this patch instead
puts the work into the callee.  A new operation::evaluate_funcall
method is added, and then this is overridden in the classes that
require special treatment.

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

	* expression.h (class operation) <evaluate_funcall>: New methods.
	* expop.h (class scope_operation) <evaluate_funcall>: New method.
	(class var_value_operation) <evaluate_funcall>: New method.
	(class structop_base_operation) <evaluate_funcall>: New method.
	(class var_msym_value_operation) <evaluate_funcall>: New method.
	(class structop_member_base): New class.
	(class structop_member_operation): Derive from
	structop_member_base.
	(class structop_mptr_operation): Derive from
	structop_member_base.
	(class funcall_operation): New class.
	* eval.c (operation::evaluate_funcall)
	(var_value_operation::evaluate_funcall)
	(scope_operation::evaluate_funcall)
	(structop_member_base::evaluate_funcall)
	(structop_base_operation::evaluate_funcall): New methods.