[llvm-pdbdump] Add support for diffing the PDB Stream.

[llvm-pdbdump] Add support for diffing the PDB Stream.

In doing so I discovered that we completely ignore some bytes
of the PDB Stream after we "finish" loading it.  These bytes
seem to specify some additional information about what kind
of data is present in the PDB.  A subsequent patch will add
code to read in those fields and store their values.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@297983 91177308-0d34-0410-b5e6-96231b3b80d8