[COFF] Expose the PE debug data directory and dump it

This directory is used to find if there is a PDB associated with an
executable. I plan to use this functionality to teach llvm-symbolizer
whether it should use DIA or DWARF to symbolize a given DLL.

Reviewers: majnemer

Differential Revision: http://reviews.llvm.org/D20885

llvm-svn: 271539