Fix throw_exception with -fno-exceptions -Werror=unused-parameter

Summary: Fix `throw_exception` with ` -fno-exceptions -Werror=unused-parameter`.

Reviewed By: yfeldblum

Differential Revision: D9693907

fbshipit-source-id: d308147b7d69cf091a9f839fda8be3464aa94b9a

folly/lang/Exception.h

@@ -30,6 +30,7 @@ namespace folly {
 template <typename Ex>
 [[noreturn]] FOLLY_NOINLINE FOLLY_COLD void throw_exception(Ex&& ex) {
 #if (__GNUC__ && !__EXCEPTIONS)
+  (void)ex;
   std::terminate();
 #else
   throw static_cast<Ex&&>(ex);