pdbdump: print out symbol names referred by publics stream.

DBI stream contains a stream number of the symbol record stream.
Symbol record streams is an array of length-type-value members.
Each member represents one symbol.

Publics stream contains offsets to the symbol record stream.
This patch is to print out all symbols that are referenced by
the publics stream.

Note that even with this patch, llvm-pdbdump cannot dump all the
information in a publics stream since it contains more information
than symbol names. I'll improve it in followup patches.

Differential Revision: http://reviews.llvm.org/D20480

llvm-svn: 270262