[OpenMP][IRBuilder] Perform finalization (incl. outlining) late

In order to fix PR44560 and to prepare for loop transformations we now
finalize a function late, which will also do the outlining late. The
logic is as before but the actual outlining step happens now after the
function was fully constructed. Once we have loop transformations we
can apply them in the finalize step before the outlining.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D74372