[llvm-pdbutil] Add the ability to explain binary files.

Using this, you can use llvm-pdbutil to export the contents of a
stream to a binary file, then run explain on the binary file so
that it treats the offset as an offset into the stream instead
of an offset into a file.  This makes it easy to compare the
contents of the same stream from two different files.

llvm-svn: 329207