Honor tags-case-fold-search during xref identifer completion

Honor tags-case-fold-search during xref identifer completion

* etc/NEWS: New entry.

* lisp/progmodes/etags.el (tags-case-fold-search):
Mark as safe-local.
(find-tag--completion-ignore-case):
Extract from tags-completion-at-point-function, find-tag-tag and
etags--xref-find-definitions.
(xref-backend-identifier-completion-ignore-case):
New method. Use it here as well.

* lisp/progmodes/xref.el
(xref-backend-identifier-completion-ignore-case): New generic.
(xref--read-identifier): Use it here.