PR21189: Teach llvm-readobj to dump bits of COFF symbol subsections required to debug using VS2012+

Reviewed at http://reviews.llvm.org/D5755
Thanks to Andrey Guskov for his help investigating this!



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220526 91177308-0d34-0410-b5e6-96231b3b80d8