[Concurrency] Remove an unnecessary 'const' from a non-static data member.

'const' breaks compilation on newer compilers.

Author: DougGregor

include/swift/ABI/Task.h

@@ -271,7 +271,7 @@ class AsyncTask : public HeapObject, public Job {
       /// Mask used for the low status bits in a wait queue item.
       static const uintptr_t statusMask = 0x03;
 
-      const uintptr_t storage;
+      uintptr_t storage;
 
       Status getStatus() const {
         return static_cast<Status>(storage & statusMask);