Pass a filename instead of a msf::WritableStream to PDBFileBuilder::commit.

WritableStream needs the exact file size to open a file, but
until we fix the final layout of a PDB file, we don't know the
size of the file.

This patch changes the parameter type of PDBFileBuilder::commit
to solve that chiecken-and-egg problem. Now the function opens
a file after fixing the layout, so it can create a file with the
exact size.

Differential Revision: https://reviews.llvm.org/D25107

llvm-svn: 282940