Fix warning on unused variable. NFC.

clang/lib/AST/Expr.cpp

@@ -1678,7 +1678,7 @@ MemberExpr *MemberExpr::Create(
   MemberExpr *E = new (Mem) MemberExpr(Base, IsArrow, OperatorLoc, MemberDecl,
                                        NameInfo, T, VK, OK, NOUR);
 
-  if (FieldDecl *Field = dyn_cast<FieldDecl>(MemberDecl)) {
+  if (isa<FieldDecl>(MemberDecl)) {
     DeclContext *DC = MemberDecl->getDeclContext();
     // dyn_cast_or_null is used to handle objC variables which do not
     // have a declaration context.