Change value::contents to be a unique_xmalloc_ptr

Change value::contents to be a unique_xmalloc_ptr

This changes value::contents to be a unique_xmalloc_ptr, removing a
small bit of manual memory management.

gdb/ChangeLog
2018-04-06  Tom Tromey  <tom@tromey.com>

	* value.c (~value): Update.
	(struct value) <contents>: Now unique_xmalloc_ptr.
	(value_contents_bits_eq, allocate_value_contents)
	(value_contents_raw, value_contents_all_raw)
	(value_contents_for_printing, value_contents_for_printing_const)
	(set_value_enclosing_type): Update.