[llvm-nm] Fix Bug 41353 - unique symbols printed as D instead of u

Summary:
https://bugs.llvm.org/show_bug.cgi?id=41353

I'm new to LLVM and C++ so please do not hesitate to iterate with me on this fix.

Patch by Mike Pozulp!

Reviewers: rupprecht, zbrid, grimar, jhenderson

Reviewed By: rupprecht, jhenderson

Subscribers: jhenderson, chrisjackson, MaskRay, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61117

llvm-svn: 361595