Do not pass a superblock to PDBFileBuilder.

When we create a PDB file using PDBFileBuilder, the information
in the superblock, such as the size of the resulting file, is not
available.

Previously, PDBFileBuilder::initialize took a superblock assuming
that all the members of the struct are correct. That is useful when
you want to restore the exact information from a YAML file, but
that's probably the only use case in which that is useful.
When we are creating a PDB file on the fly, we have to backfill the
members.

This patch redefines PDBFileBuilder::initialize to take only a
block size. Now all the other members are left as default values,
so that they'll be updated when commit() is called.

Differential Revision: https://reviews.llvm.org/D25108

llvm-svn: 282944