Added versions of ReadPtr that takes an explicit SerializedPtrID. This allows

clients of the Deserializer to read the pointer ID before they are ready
to deserialize the object (which can mean registering a pointer reference
with the backpatcher).

Changed some methods that took an argument "SerializedPtrID" to "const SerializedPtrID&" (pass-by-reference).  This is to accommodate a future
revision of SerializedPtrID where it may be much fatter than an unsigned
integer.

llvm-svn: 44021