Speed up xref rendering for matches on very long lines

Speed up xref rendering for matches on very long lines

* lisp/progmodes/xref.el (xref--insert-xrefs): Cut up the current
line into pieces here for multiple matches's summaries, so that
xref--insert-xrefs can do less work (bug#46859).
(xref--insert-xrefs): Do less work.
(xref--outdated-p):
Update accordingly to how the summary creation logic changed.
(xref--buf-pairs-iterator): Update to the new calling convention.
(xref-location-column): Effectively rename back to
xref-file-location-column since the generic version is now unused.

* test/lisp/progmodes/xref-tests.el
(xref-matches-in-directory-finds-two-matches-on-the-same-line)
(xref-matches-in-directory-finds-an-empty-line-regexp-match):
Adjust to the xref-location-column change.
(xref-matches-in-files-trims-summary-for-matches-on-same-line):
New test.

* test/lisp/progmodes/xref-resources/file1.txt:
Change contents slightly to test the new xref--outdated-p code.