Bytecode Reader Cleanup:

- provide the correct conversion for ModuleBlockID in read_block (a potential
  bug but not actually exploited because reading module block ids doesn't
  use read_block).
- install support for handleTypeList handler
- install support for handleDependentLibrary handler
- install support for handleTargetTriple handler
- clean up comments, output strings,
- ensure that processing function arguments doesn't SIGSEGV if one of the
  arguments is a null pointer (yeah, it happened .. weird)
- prepare for version 5 bytecode by documenting what will change.

llvm-svn: 15981