Return unique_xmalloc_ptr from call_site_find_chain

call_site_find_chain returns a pointer that the caller must
deallocate.  It seemed better here to return a unique_xmalloc_ptr
instead.

gdb/ChangeLog
2020-02-14  Tom Tromey  <tromey@adacore.com>

	* dwarf2/frame-tailcall.c (dwarf2_tailcall_sniffer_first):
	Update.
	* dwarf2/loc.h (call_site_find_chain): Return unique_xmalloc_ptr.
	* dwarf2/loc.c (call_site_find_chain_1): Return
	unique_xmalloc_ptr.
	(call_site_find_chain): Likewise.