Export class symbols.

Apparently some people prefer to call make-instance directly instead
of using constructor functions.

* xref.el (xref-file-location): Renamed from xref--file-location.
(xref-buffer-location): Renamed from xref--buffer-location.
(xref-bogus-location): Renamed from xref--bogus-location.
(xref-make-file-location): Renamed from xref-file-location.
(xref-make-buffer-location): Renamed from xref-buffer-location.
(xref-make-bogus-location): Renamed from xref-bogus-location.