...with the aid of an extended compilerplugins/clang/store/sfxitemsetrewrite.cxx
(which in turn needed a small addition to compilerplugins/clang/check.hxx).

Enable svl::detail::validGap check for the static case, but keep it disabled for
now for the dynamic case.

Change-Id: I4846ba8e99aff94a86518e2cb5044e575093386e