Fix c-tentative-buffer-changes to be nestable in c-save-buffer-state

Fix c-tentative-buffer-changes to be nestable in c-save-buffer-state

* lisp/progmodes/cc-defs.el (c-tentative-buffer-changes)
(c-tnt-chng-record-state, c-tnt-chng-cleanup): Enhance such that a
buffer-undo-list of t is handled specially, so that a nil isn't consed onto
it.  Thus garbage collection can't later remove the (nil . t) from the end of
the buffer-undo-list, causing an infinite loop.