ABI: use a c-style cast in the header (NFC)

Use a C-style cast to appease cl depending on the template type
parameters used.  It does not like a reinterpret_cast between integral
types.

Author: compnerd

include/swift/ABI/Metadata.h

@@ -1645,7 +1645,11 @@ class TargetProtocolDescriptorRef {
     assert(!isObjC());
 #endif
 
-    return reinterpret_cast<ProtocolDescriptorPointer>(storage & ~IsObjCBit);
+    // NOTE: we explicitly use a C-style cast here because cl objects to the
+    // reinterpret_cast from a uintptr_t type to an unsigned type which the
+    // Pointer type may be depending on the instantiation.  Using the C-style
+    // cast gives us a single path irrespective of the template type parameters.
+    return (ProtocolDescriptorPointer)(storage & ~IsObjCBit);
   }
 
   /// Retrieve the raw stored pointer and discriminator bit.