Add get_clone_versions and get_target_version functions.

Add get_clone_versions and get_target_version functions.

This is a reimplementation of get_target_clone_attr_len,
get_attr_str, and separate_attrs using string_slice and auto_vec to make
memory management and use simpler.

Adds get_target_version helper function to get the target_version string
from a decl.

gcc/c-family/ChangeLog:

	* c-attribs.cc (handle_target_clones_attribute): Change to use
	get_clone_versions.

gcc/ChangeLog:

	* tree.cc (get_clone_versions): New function.
	(get_clone_attr_versions): New function.
	(get_version): New function.
	* tree.h (get_clone_versions): New function.
	(get_clone_attr_versions): New function.
	(get_target_version): New function.