[llvm-pdbdump] Add support for printing source files and compilands.

This adds two command line options to llvm-pdbdump.

--source-files prints a flat list of all source files in the PDB.

--compilands prints a list of all compilands (e.g. object files)
             that the PDB knows about, and for each one, a list of
             source files that the compiland is composed of as well
             as a hash of the original source file.

llvm-svn: 227276