[llvm-nm] Report '.comment' ELF sections as 'n' instead of '?'

Summary:
The previous implementation reported `.comment` sections as '?'
GNU uses 'n' which means  "The symbol is a debugging symbol."  `.note`  sections are represented as 'n' too.

The test related to this change was updated to CHECK-NEXT to ensure
order and that we did not miss any symbols in the dump.

Reviewers: jhenderson

Reviewed By: jhenderson

Subscribers: rupprecht, llvm-commits

Differential Revision: https://reviews.llvm.org/D57544

llvm-svn: 352891