[pdbutil] Add a command to dump the FPM.

Recently problems have been discovered in the way we write the FPM
(free page map).  In order to fix this, we first need to establish
a baseline about what a correct FPM looks like using an MSVC
generated PDB, so that we can then make our own generated PDBs
match.  And in order to do this, the dumper needs a mode where it
can dump an FPM so that we can write tests for it.

This patch adds a command to dump the FPM, as well as a test against
a known-good PDB.

llvm-svn: 309894