Fix a few small issues in llvm-pdbutil

Running "llvm-pdbutil dump -all" on linux (using the native PDB reader),
over a few PDBs pulled from the Microsoft public symbol store uncovered 
a few small issues:

- stripped PDBs might not have the strings stream (/names)
- stripped PDBs might not have the "module info" stream

Differential Revision: https://reviews.llvm.org/D54006

llvm-svn: 346010