Additional content describing variable bit rate encoding, correcting the

Additional content describing variable bit rate encoding, correcting the
definition of the signature block and module block, and adding definitions
of the global type pool and symbol table. More still to come.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@13658 91177308-0d34-0410-b5e6-96231b3b80d8