bug#31027: 27.0.50; xref, tags-location-ring equivalent

[Charles A. Roelli]

tags-location-ring seems to have no replacement in xref.el.

from etags.el:
(defvar tags-location-ring (make-ring xref-marker-ring-length)
  "Ring of markers which are locations visited by \\[find-tag].
Pop back to the last location with \\[negative-argument] \\[find-tag].")

We should add a "xref-location-ring" (or similar) that stores the
locations visited by "xref-find-definitions", and we can allow jumping
to them with C-u - M-., in the same way as "find-tag" does.