[OpenMP] Replace ExternalizationRAII with virtual uses

The externalization was always a stopgap solution. One of the drawbacks
is that it is very conservative no matter if we actually require the
functions at the end of the pass. The new concept is more generic and
properly integrates into the dependence graph. Whenever we might need a
function, it has a "virtual use" that cannot be analyzed. If we do not
because of some AA state, there will be a dependence to ensure state
changes trigger revisits of uses, including a potentially new virtual
use.