[flang] Fix warning in build (#100134)

After tblah's update #99817, I'm getting a warning about an
unusedvariable. It looks to me like the warning is bogus, possibly due
to a bug in the compiler I'm using (GCC 9.3.0). But adding a
"maybe_unused" clause fixes it and makes my builds clean.

flang/lib/Lower/DirectivesCommon.h

@@ -137,7 +137,8 @@ static inline void genOmpAtomicHintAndMemoryOrderClauses(
 }
 
 template <typename AtomicListT>
-static void processOmpAtomicTODO(mlir::Type elementType, mlir::Location loc) {
+static void processOmpAtomicTODO(mlir::Type elementType,
+                                 [[maybe_unused]] mlir::Location loc) {
   if (!elementType)
     return;
   if constexpr (std::is_same<AtomicListT,