[llvm-pdbdump] Allow printing only a portion of a stream.

When dumping raw data from a stream, you might know the offset
of a certain record you're interested in, as well as how long
that record is.  Previously, you had to dump the entire stream
and wade through the bytes to find the interesting record.

This patch allows you to specify an offset and length on the
command line, and it will only dump the requested range.

llvm-svn: 301607