Implement UDL changes from P0732R2.

Implement UDL changes from P0732R2.

Implementing the UDL changes was pretty straightforward; I simplified
cp_parser_userdef_string_literal using the releasing_vec type from mangle.c.

While looking at this, I realized that the string UDL template taking a
character pack that we implemented for C++14 didn't actually make it into
C++14, so I've added a pedwarn for it and no longer suggest it in the
diagnostic about an invalid UDL template.

	* cp-tree.h (struct releasing_vec): Move from mangle.c.
	Add get_ref method.
	* parser.c (cp_parser_userdef_string_literal): Use it.  Handle
	passing the string to a single template parameter of class type.
	(cp_parser_template_declaration_after_parameters): Allow it.
	Pedwarn about the character pack template that was proposed but not
	accepted for C++14, and don't suggest it.

From-SVN: r265790