Fix demangler to handle conversion operators correctly.

libiberty/
	PR other/59195
	* cp-demangle.c (struct d_info_checkpoint): New struct.
	(struct d_print_info): Add current_template field.
	(d_operator_name): Set flag when processing a conversion
	operator.
	(cplus_demangle_type): When processing <template-args> for
	a conversion operator, backtrack if necessary.
	(d_expression_1): Renamed from d_expression.
	(d_expression): New wrapper around d_expression_1.
	(d_checkpoint): New function.
	(d_backtrack): New function.
	(d_print_init): Initialize current_template.
	(d_print_comp): Set current_template.
	(d_print_cast): Put current_template in scope for
	printing conversion operator name.
	(cplus_demangle_init_info): Initialize is_expression and
	is_conversion.
	* cp-demangle.h (struct d_info): Add is_expression and
	is_conversion fields.
	* testsuite/demangle-expected: New test cases.

From-SVN: r205292