[pdb] Use MsfBuilder to handle the writing PDBs.

Previously we would read a PDB, then write some of it back out,
but write the directory, super block, and other pertinent metadata
back out unchanged.  This generates incorrect PDBs since the amount
of data written was not always the same as the amount of data read.

This patch changes things to use the newly introduced `MsfBuilder`
class to write out a correct and accurate set of Msf metadata for
the data *actually* written, which opens up the door for adding and
removing type records, symbol records, and other types of data to
an existing PDB.

llvm-svn: 275627