[pdb/lld] Write a valid FPM.

The PDB reserves certain blocks for the FPM that describe which
blocks in the file are allocated and which are free.  We weren't
filling that out at all, and in some cases we were even stomping
it with incorrect data.  This patch writes a correct FPM.

Differential Revision: https://reviews.llvm.org/D36235

llvm-svn: 309896