Updated Deserializer class to provide more information about the current

Updated Deserializer class to provide more information about the current
block that is being visited in the bitstream.  The client can also now
skip blocks before reading them, and query the current abbreviation number
as seen from the perspective of the Deserializer.  This allows the client
to be more interactive in the deserialization process (if they so choose).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@43916 91177308-0d34-0410-b5e6-96231b3b80d8