[flang] Add genEval to the AbstractConverter (#75140)

There was some discussion on discourse[1] about allowing call to FIR
generation functions from other part of lowering belonging to OpenMP.

This solution exposes a simple `genEval` member function on the
`AbstractConverter` so that IR generation for PFT Evaluation objects can
be called from lowering outside of the FirConverter but not exposing it.

[1] https://discourse.llvm.org/t/openmp-lowering-from-pft-to-fir/75263