Changes to the V2 bytecode format:

  - Null values are implicitly encoded instead of explicitly, this makes
    things more compact!
  - More compactly represent ConstantPointerRefs
  - Bytecode files are represented as:
      Header|GlobalTypes|GlobalVars/Function Protos|Constants|Functions|SymTab
    instead of
      Header|GlobalTypes|Constants|GlobalVars/Function Protos|Functions|SymTab
    which makes a lot of things simpler.

Writer changes:
  - We now explictly encode versioning information in the bytecode files.
  - This allows new code to read bytecode files produced by old code, but
    new bytecode files can have enhancements such as the above.  Although this
    makes the reader a bit more complex (having to deal with old formats), the
    writer only needs to be able to produce the most recent version.

llvm-svn: 5749