[pdb] Teach MsfBuilder and other classes about the Free Page Map.

Block 1 and 2 of an MSF file are bit vectors that represent the
list of blocks allocated and free in the file.  We had been using
these blocks to write stream data and other data, so we mark them
as the free page map now.  We don't yet serialize these pages to
the disk, but at least we make a note of what it is, and avoid
writing random data to them.

Doing this also necessitated cleaning up some of the tests to be
more general and hardcode fewer values, which is nice.

llvm-svn: 275629

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