SemaOpenMP.cpp: Fix a warning in r258543. [-Wunused-variable]

llvm-svn: 258598
diff --git a/clang/lib/Sema/SemaOpenMP.cpp b/clang/lib/Sema/SemaOpenMP.cpp
index 3157da4..6272a62 100644
--- a/clang/lib/Sema/SemaOpenMP.cpp
+++ b/clang/lib/Sema/SemaOpenMP.cpp
@@ -9007,6 +9007,7 @@
     auto *FD = dyn_cast<FieldDecl>(D);
 
     assert((VD || FD) && "Only variables or fields are expected here!");
+    (void)FD;
 
     // OpenMP 4.5 [2.15.5.1, map Clause, Restrictions, p.10]
     //  threadprivate variables cannot appear in a map clause.