Use block_symbol in var_value_operation

Use block_symbol in var_value_operation

I noticed that var_value_operation takes a block and a symbol, and
most callers destructure a block_symbol to pass in.  It seems better
for this class to simply hold a block_symbol instead.

Tested on x86-64 Fedora 32.

gdb/ChangeLog
2021-04-15  Tom Tromey  <tromey@adacore.com>

	* rust-exp.y (rust_parser::convert_ast_to_expression): Update.
	* parse.c (parser_state::push_symbol, parser_state::push_dollar):
	Update.
	* p-exp.y (variable): Update.
	* m2-exp.y (variable): Update.
	* go-exp.y (variable): Update.
	* expprint.c (dump_for_expression): New overload.
	* expop.h (check_objfile): New overload.
	(check_constant): New overload.
	(class var_value_operation): Use block_symbol.
	<get_symbol>: Rewrite.
	* eval.c (var_value_operation::evaluate)
	(var_value_operation::evaluate_funcall)
	(var_value_operation::evaluate_for_address)
	(var_value_operation::evaluate_for_address)
	(var_value_operation::evaluate_with_coercion)
	(var_value_operation::evaluate_for_sizeof)
	(var_value_operation::evaluate_for_cast): Update.
	* d-exp.y (PrimaryExpression): Update.
	* c-exp.y (variable): Update.
	* ax-gdb.c (var_value_operation::do_generate_ax): Update.
	* ada-lang.c (ada_var_value_operation::evaluate_for_cast)
	(ada_var_value_operation::evaluate)
	(ada_var_value_operation::resolve)
	(ada_funcall_operation::resolve): Update.
	* ada-exp.y (write_var_from_sym, write_object_renaming)
	(write_ambiguous_var, write_var_or_type, write_name_assoc)
	(maybe_overload): Update.
	* ada-exp.h (class ada_var_value_operation) <get_block>: Rewrite.