[flang] Add lowering TODO for separate module procedures

[clementval]

Summary

MODULE FUNCTION and MODULE SUBROUTINE currently cause lowering crash:
"symbol is not mapped to any IR value" because special care is needed
to handle their interface.

Add a TODO for now.

Example of program that crashed and will hit the TODO:

module mod
  interface
    module subroutine sub
    end subroutine
  end interface
contains
  module subroutine sub
    x = 42
  end subroutine
end module

This patch is part of the upstreaming effort from fir-dev branch.

Co-authored-by: Jean Perier <[e-mail]>

[jeanPerier]

LGTM

flang/lib/Lower/Bridge.cpp

createGlobalOutsideOfFunctionLowering(
         [&]() { Fortran::lower::defineCommonBlocks(*this, commonBlocks); });
   }
 
   /// Lower a procedure (nest).
   void lowerFunc(Fortran::lower::pft::FunctionLikeUnit &funit) {
     if (!funit.isMainProgram()) {
       const Fortran::semantics::Symbol &procSymbol =
           funit.getSubprogramSymbol();
-      if (procSymbol.owner().IsSubmodule()) {
+      if (procSymbol.owner().IsSubmodule())
         TODO(toLocation(), "support for submodules");
-        return;
-      }
+      if (Fortran::semantics::IsSeparateModuleProcedureInterface(&procSymbol))
+        TODO(toLocation(), "separate module procedure");
     }
     setCurrentPosition(funit.getStartingSourceLoc());
     for (int entryIndex = 0, last = funit.entryPointList.size();
          entryIndex < last; ++entryIndex) {
       funit.setActiveEntry(entryIndex);
       startNewFunction(funit); // the entry point for lowering this procedure
       for (Fortran::lower::pft::Evaluation &eval : funit.evaluationList)
         genFIR(eval);