[llvm-readobj] [COFF] Print the symbol index for relocations

[llvm-readobj] [COFF] Print the symbol index for relocations

There can be multiple local symbols with the same name (for e.g.
comdat sections), and thus the symbol name itself isn't enough
to disambiguate symbols.

Differential Revision: https://reviews.llvm.org/D56140

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350288 91177308-0d34-0410-b5e6-96231b3b80d8