[llvm-pdbdump] [yaml2pdb] always include object file name in module info

Summary:
Previously, the yaml2pdb subcommand of llvm-pdbdump only
included object file names in module info if a module info stream was
present. This change makes it so that we include the object file name
even if there is no module info stream for the module. As a result,
running
llvm-pdbdump pdb2yaml -dbi-module-info original.pdb > original.yaml &&
llvm-pdbdump yaml2pdb -pdb=new.pdb original.yaml && llvm-pdbdump
pdb2yaml -dbi-module-info new.pdb > new.yaml now produces identical
original.yaml and new.yaml files.

Reviewers: amccarth, zturner

Reviewed By: zturner

Subscribers: fhahn, llvm-commits

Differential Revision: https://reviews.llvm.org/D33463

llvm-svn: 303891