xml-support.c: Use std::vector

xml-support.c: Use std::vector

scope_level::scope_level needed both a move ctor and a dtor explicitly
coded, but those will be eliminated in a following patch.

gdb/ChangeLog:
2017-04-18  Pedro Alves  <palves@redhat.com>

	* xml-support.c: Include <vector>.
	(scope_level::scope_level(const gdb_xml_element *))
	(scope_level::scope_level(scope_level&&)): New.
	(scope_level::~scope_level): New.
	(scope_level_s): Delete.
	(gdb_xml_parser::scopes): Now a std::vector.
	(gdb_xml_body_text, gdb_xml_start_element, gdb_xml_end_element):
	Use std::vector.
	(gdb_xml_parser::~gdb_xml_parser): Remove now unnecessary
	scope cleanup code.
	(gdb_xml_parser::gdb_xml_parser): Remove explicit initialization
	of the scopes member.  Use std::vector::emplace_back.