Fix Flymake's user-visible accessors of diagnostic positions

Fix Flymake's user-visible accessors of diagnostic positions

Diagnostics are supported by overlays, and they can legitimately move
around.  So flymake-diagnostic-beg and flymake-diagnostic-end must
look up the overlay positions, not the immutable slots of the
flymake--diag structure, which become stale.

* lisp/progmodes/flymake.el (version): Bump to 1.0.8.
(flymake-diagnostic-beg, flymake-diagnostic-end): Use diag's
overlay.
(flymake-show-diagnostic): Use flymake-diagnostic-end,
flymake-diagnostic-beg.