[llvm-pdbutil] Add an export subcommand.

This command can dump the binary contents of a stream to a file.
This is useful when you want to do side-by-side comparisons of
a specific stream from two PDBs to examine the differences between
them.  You can export both of them to a file, then open them up
side by side in a hex editor (for example), so as to eliminate any
differences that might arise from the contents being on different
blocks in the PDB.

In subsequent patches I plan to improve the "explain" subcommand
so that you can explain the contents of a binary file that isn't
necessarily a full PDB, but one of these dumped streams, by telling
the subcommand how to interpret the contents.

llvm-svn: 329002