shims: convert _dispatch_preemption_yeild to an expression

Because `_dispatch_preemption_yield` is used an expression as: ~~~ ((void) (q), _dispatch_preemption_yield(n)) ~~~ we need to convert the expansion to an expression. We _could_ also just wrap the invocation into a parenthesis as an alternative approach, but this should be safe enough to let us get away with it.

src/shims/yield.h

@@ -161,14 +161,14 @@ void *_dispatch_wait_for_enqueuer(void **ptr, void **tailp);
 #define _dispatch_preemption_yield_to(th, n) thread_switch(th, \
 DISPATCH_YIELD_THREAD_SWITCH_OPTION, (mach_msg_timeout_t)(n))
 #elif HAVE_PTHREAD_YIELD_NP
-#define _dispatch_preemption_yield(n) { (void)n; pthread_yield_np(); }
-#define _dispatch_preemption_yield_to(th, n) { (void)n; pthread_yield_np(); }
+#define _dispatch_preemption_yield(n) ({ (void)n; pthread_yield_np(); })
+#define _dispatch_preemption_yield_to(th, n) ({ (void)n; pthread_yield_np(); })
 #elif defined(_WIN32)
-#define _dispatch_preemption_yield(n) { (void)n; Sleep(0); }
-#define _dispatch_preemption_yield_to(th, n) { (void)n; Sleep(0); }
+#define _dispatch_preemption_yield(n) ({ (void)n; Sleep(0); })
+#define _dispatch_preemption_yield_to(th, n) ({ (void)n; Sleep(0); })
 #else
-#define _dispatch_preemption_yield(n) { (void)n; sched_yield(); }
-#define _dispatch_preemption_yield_to(th, n) { (void)n; sched_yield(); }
+#define _dispatch_preemption_yield(n) ({ (void)n; sched_yield(); })
+#define _dispatch_preemption_yield_to(th, n) ({ (void)n; sched_yield(); })
 #endif // HAVE_MACH
 
 #if DISPATCH_HAVE_YIELD_TO_ENQUEUER