[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

Author: clementval

flang/include/flang/Lower/AbstractConverter.h

@@ -280,6 +280,10 @@ class AbstractConverter {
   // Miscellaneous
   //===--------------------------------------------------------------------===//
 
+  /// Generate IR for Evaluation \p eval.
+  virtual void genEval(pft::Evaluation &eval,
+                       bool unstructuredContext = true) = 0;
+
   /// Return options controlling lowering behavior.
   const Fortran::lower::LoweringOptions &getLoweringOptions() const {
     return loweringOptions;

flang/lib/Lower/Bridge.cpp

@@ -839,6 +839,11 @@ class FirConverter : public Fortran::lower::AbstractConverter {
     }
   }
 
+  void genEval(Fortran::lower::pft::Evaluation &eval,
+               bool unstructuredContext) override final {
+    genFIR(eval, unstructuredContext);
+  }
+
   //===--------------------------------------------------------------------===//
   // Utility methods
   //===--------------------------------------------------------------------===//