Added typedef "SerializedPtrID" to represent the pointer handle written to disk

instead of just using "unsigned".  This gives us more flexibility in changing
the definition of the handle later, and is more self-documenting.

Added tracking of block stack in the Deserializer.  Now clients can query
if they are still within a block using the methods GetCurrentBlockLocation()
and FinishedBlock().

llvm-svn: 43903