[flang][openacc] Switch to use TODO from D88909

[clementval]

Summary

Use the Todo.h header file introduce in D88909 to marke part of the lowering that are
not done yet.

[jeanPerier]

LGTM

[clementval]

Rebase

flang/lib/Lower/OpenACC.cpp

 //===----------------------------------------------------------------------===//
 
 #include "flang/Lower/OpenACC.h"
 #include "flang/Common/idioms.h"
 #include "flang/Lower/Bridge.h"
 #include "flang/Lower/FIRBuilder.h"
 #include "flang/Lower/PFTBuilder.h"
 #include "flang/Lower/Support/BoxValue.h"
+#include "flang/Lower/Todo.h"
 #include "flang/Parser/parse-tree.h"
 #include "flang/Semantics/tools.h"
 #include "mlir/Dialect/OpenACC/OpenACC.h"
 #include "llvm/Frontend/OpenACC/ACC.h.inc"
 
-#define TODO() llvm_unreachable("not yet implemented")
-
 static const Fortran::parser::Name *
 getDesignatorNameIfDataRef(const Fortran::parser::Designator &designator) {
   const auto *dataRef{std::get_if<Fortran::parser::DataRef>(&designator.u)};
   return dataRef ? std::get_if<Fortran::parser::Name>(&dataRef->u) : nullptr;
 }
 
 static void genObjectList(const Fortran::parser::AccObjectList &objectList,
                           Fortran::lower::AbstractConverter &converter,
void Fortran::lower::genOpenACCConstruct(
     const Fortran::parser::OpenACCConstruct &accConstruct) {
 
   std::visit(
       common::visitors{
           [&](const Fortran::parser::OpenACCBlockConstruct &blockConstruct) {
             genACC(converter, eval, blockConstruct);
           },
           [&](const Fortran::parser::OpenACCCombinedConstruct
-                  &combinedConstruct) { TODO(); },
+                  &combinedConstruct) {
+            TODO("OpenACC Combined construct not lowered yet!");
+          },
           [&](const Fortran::parser::OpenACCLoopConstruct &loopConstruct) {
             genACC(converter, eval, loopConstruct);
           },
           [&](const Fortran::parser::OpenACCStandaloneConstruct
-                  &standaloneConstruct) { TODO(); },
+                  &standaloneConstruct) {
+            TODO("OpenACC Standalone construct not lowered yet!");
+          },
           [&](const Fortran::parser::OpenACCRoutineConstruct
-                  &routineConstruct) { TODO(); },
+                  &routineConstruct) {
+            TODO("OpenACC Routine construct not lowered yet!");
+          },
           [&](const Fortran::parser::OpenACCCacheConstruct &cacheConstruct) {
-            TODO();
+            TODO("OpenACC Cache construct not lowered yet!");
           },
           [&](const Fortran::parser::OpenACCWaitConstruct &waitConstruct) {
-            TODO();
+            TODO("OpenACC Wait construct not lowered yet!");
           },
           [&](const Fortran::parser::OpenACCAtomicConstruct &atomicConstruct) {
-            TODO();
+            TODO("OpenACC Atomic construct not lowered yet!");
           },
       },
       accConstruct.u);
 }