[Builders.h] Silence a warning by adding a cast.

[lattner]

Summary

The no-result version of createOrFold calls 'tryFold' but
ignores the result since it doesn't matter what it produced.
Explicitly cast to void to silence this warning:

../llvm/mlir/include/mlir/IR/Builders.h:454:5: warning: ignoring return value of function declared with 'nodiscard' attribute [-Wunused-result]

tryFold(op.getOperation(), unused);
^~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~

mlir/include/mlir/IR/Builders.h

public:
 
   /// Overload to create or fold a zero result operation.
   template <typename OpTy, typename... Args>
   typename std::enable_if<OpTy::template hasTrait<OpTrait::ZeroResult>(),
                           OpTy>::type
   createOrFold(Location location, Args &&...args) {
     auto op = create<OpTy>(location, std::forward<Args>(args)...);
     SmallVector<Value, 0> unused;
-    tryFold(op.getOperation(), unused);
+    (void)tryFold(op.getOperation(), unused);
 
     // Folding cannot remove a zero-result operation, so for convenience we
     // continue to return it.
     return op;
   }
 
   /// Attempts to fold the given operation and places new results within
   /// 'results'. Returns success if the operation was folded, failure otherwise.