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

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);
    ^~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~

Differential Revision: https://reviews.llvm.org/D111951