Make cp_comp_to_string return a gdb::unique_xmalloc_ptr<char>

Make cp_comp_to_string return a gdb::unique_xmalloc_ptr<char>

To help avoid issues like the one fixed by e88e8651cf34 ("Fix memory
leak in cp-support.c").

gdb/ChangeLog:
2017-08-09  Pedro Alves  <palves@redhat.com>

	* cp-name-parser.y (cp_comp_to_string): Return a
	gdb::unique_xmalloc_ptr<char>.
	* cp-support.c (replace_typedefs_qualified_name)
	(replace_typedefs): Adjust to use gdb::unique_xmalloc_ptr<char>.
	(cp_canonicalize_string_full): Use op= instead of explicit
	convertion.
	(cp_class_name_from_physname, method_name_from_physname)
	(cp_func_name, cp_remove_params): Adjust to use
	gdb::unique_xmalloc_ptr<char>.
	* cp-support.h (cp_comp_to_string): Return a
	gdb::unique_xmalloc_ptr<char>.
	* python/py-type.c (typy_lookup_type): Adjust to use
	gdb::unique_xmalloc_ptr<char>.