Replace VEC(gdb_xml_value_s) with std::vector

Replace VEC(gdb_xml_value_s) with std::vector

This patch replaces VEC(gdb_xml_value_s), which is passed to XML
visitors, with an std::vector.  In order to be able to remove the
cleanup in gdb_xml_parser::start_element, the gdb_xml_parser structure
is made to own the value with a gdb::unique_xmalloc_ptr.

This patch has been tested on the buildbot.

gdb/ChangeLog:

	* xml-support.h (struct gdb_xml_value): Add constructor.
	<value>: Change type to unique_xmalloc_ptr.
	(gdb_xml_value_s): Remove typedef.
	(DEF_VEC_O (gdb_xml_value_s)): Remove.
	(gdb_xml_element_start_handler): Change parameter type to
	std::vector.
	(xml_find_attribute): Likewise.
	* xml-support.c (xml_find_attribute): Change parameter type to
	std::vector and adjust.
	(gdb_xml_values_cleanup): Remove.
	(gdb_xml_parser::start_element): Adjust to std::vector.
	(xinclude_start_include): Change paraeter type to std::vector
	and adjust.
	* btrace.c (check_xml_btrace_version): Likewise.
	(parse_xml_btrace_block): Likewise.
	(parse_xml_btrace_pt_config_cpu): Likewise.
	(parse_xml_btrace_pt): Likewise.
	(parse_xml_btrace_conf_bts): Likewise.
	(parse_xml_btrace_conf_pt): Likewise.
	* memory-map.c (memory_map_start_memory): Likewise.
	(memory_map_start_property): Likewise.
	* osdata.c (osdata_start_osdata): Likewise.
	(osdata_start_item): Likewise.
	(osdata_start_column): Likewise.
	* remote.c (start_thread): Likewise.
	* solib-aix.c (library_list_start_library): Likewise.
	(library_list_start_list): Likewise.
	* solib-svr4.c (library_list_start_library): Likewise.
	(svr4_library_list_start_list): Likewise.
	* solib-target.c (library_list_start_segment): Likewise.
	(library_list_start_section): Likewise.
	(library_list_start_library): Likewise.
	(library_list_start_list): Likewise.
	* tracepoint.c (traceframe_info_start_memory): Likewise.
	(traceframe_info_start_tvar): Likewise.
	* xml-syscall.c (syscall_start_syscall): Likewise.
	* xml-tdesc.c (tdesc_start_target): Likewise.
	(tdesc_start_feature): Likewise.
	(tdesc_start_reg): Likewise.
	(tdesc_start_union): Likewise.
	(tdesc_start_struct): Likewise.
	(tdesc_start_flags): Likewise.
	(tdesc_start_enum): Likewise.
	(tdesc_start_field): Likewise.
	(tdesc_start_enum_value): Likewise.
	(tdesc_start_vector): Likewise.