Runtime: make padding explicit for ElementStorage

The `ElementStorage` member `Capacity` will be padded up to the next pointer alignment due to the storage of the `ElemTy` by C's struct layout rules.  However, this is implicit and not entirely guaranteed.  Instead, make the storage a pointer sized value, and then truncate to 32-bits to maintain ABI compatibility.  This becomes important as we expand on the runtime metadata handling.

Author: compnerd

include/swift/Runtime/Concurrent.h

@@ -812,7 +812,7 @@ struct ConcurrentReadableHashMap {
   /// the first element of a variable-length array, whose size is determined by
   /// the allocation.
   struct ElementStorage {
-    uint32_t Capacity;
+    uintptr_t Capacity : 32;
     ElemTy Elem;
 
     static ElementStorage *allocate(size_t capacity) {