[Serialization] Compact the Offset value in PartiallySerialized

serialization::BitOffset is set up for being in a PointerUnion, but in
this case that's not just overkill but actually wasteful, since we
have an extra flag to pack in. Use a raw bitfield instead.

No functionality change.

include/swift/Serialization/ModuleFile.h

@@ -241,7 +241,7 @@ public:
     T Value;
 
     /// The offset.
-    serialization::BitOffset Offset;
+    unsigned Offset : 31;
 
     unsigned IsFullyDeserialized : 1;