New top-level buffer-local value functions

New top-level buffer-local value functions

* src/eval.c (local_toplevel_binding)
(Fbuffer_local_toplevel_value, Fset_buffer_local_toplevel_value)
(syms_of_eval): New functions.
* doc/lispref/variables.texi (Default Value):
* lisp/subr.el (setq-local):
* etc/NEWS: Document them.
* test/lisp/emacs-lisp/lisp-tests.el
(core-elisp-tests-4-toplevel-values): New test.
* lisp/progmodes/xref.el (xref--show-xref-buffer)
(xref-show-definitions-buffer-at-bottom):
* lisp/vc/vc-dispatcher.el (vc-setup-buffer):
Use set-buffer-local-toplevel-value.

(cherry picked from commit 45627ca7ccd0e84452d497997a7f75535ed83864)