[pdb] Print out file names instead of file offsets.

When printing line information and file checksums, we were printing
the file offset field from the struct header.  This teaches
llvm-pdbdump how to turn those numbers into the filename.  In the
case of file checksums, this is done by looking in the global
string table.  In the case of line contributions, this is done
by indexing into the file names buffer of the DBI stream.  Why
they use a different technique I don't know.

llvm-svn: 271630