[llvm-pdbutil] Dig deeper into the PDB and DBI streams when explaining.

This will show more detail when using `llvm-pdbutil explain` on an
offset in the DBI or PDB streams.  Specifically, it will dig into
individual header fields and substreams to give a more precise
description of what the byte represents.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@328878 91177308-0d34-0410-b5e6-96231b3b80d8