[Frontend][OpenMP] Reduction modifier must be applied somewhere (#92160)

Detect the case when a reduction modifier ends up not being applied
after construct decomposition, treat it as an error.

This fixes a regression in the gfortran test suite after PR90098.

Author: kparzysz

flang/test/Lower/OpenMP/invalid-reduction-modifier.f90

@@ -0,0 +1,14 @@
+!Remove the --crash below once we can diagnose the issue more gracefully.
+!RUN: not --crash %flang_fc1 -fopenmp -emit-hlfir -o - %s
+
+! Check that we reject the "task" reduction modifier on the "simd" directive.
+
+subroutine fred(x)
+  integer, intent(inout) :: x
+
+  !$omp simd reduction(task, +:x)
+  do i = 1, 100
+    x = foo(i)
+  enddo
+  !$omp end simd
+end

llvm/include/llvm/Frontend/OpenMP/ConstructDecompositionT.h

@@ -935,7 +935,8 @@ bool ConstructDecompositionT<C, H>::applyClause(
       // Apply clause without modifier.
       leaf.clauses.push_back(unmodified);
     }
-    applied = true;
+    // The modifier must be applied to some construct.
+    applied = effectiveApplied;
   }
 
   if (!applied)