Fix a few small issues in llvm-pdbutil

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346010 91177308-0d34-0410-b5e6-96231b3b80d8