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

[mattd]

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.

[jhenderson]

LGTM. The CHECK-NEXT can be in a follow-on, or just during the commit, assuming there's no issues.

llvm/test/Object/nm-trivial-object.test

118–127

Could you use ELF64-DEBUG-SYMS-NEXT here to capture all the section names, and make sure there are no others we're missing?

llvm/test/Object/nm-trivial-object.test

 WEAK-ELF64:                  w f1
 WEAK-ELF64: 0000000000000000 W f2
 WEAK-ELF64:                  v x1
 WEAK-ELF64: 0000000000000000 V x2
 
 ABSOLUTE-ELF64: 0000000000000123 a a1
 ABSOLUTE-ELF64: 0000000000000123 A a2
 
 ELF64-DEBUG-SYMS: 00000000 b .bss
+ELF64-DEBUG-SYMS: 00000000 n .comment
 ELF64-DEBUG-SYMS: 00000000 d .data
 ELF64-DEBUG-SYMS: 00000000 N .debug_abbrev
 ELF64-DEBUG-SYMS: 00000000 N .debug_aranges
 ELF64-DEBUG-SYMS: 00000000 N .debug_frame
 ELF64-DEBUG-SYMS: 00000000 N .debug_info
 ELF64-DEBUG-SYMS: 00000000 N .debug_line
 ELF64-DEBUG-SYMS: 00000000 N .debug_pubnames
 ELF64-DEBUG-SYMS: 00000000 n .note.GNU-stack
 ELF64-DEBUG-SYMS: 00000000 t .text
 ELF64-DEBUG-SYMS: 00000000 a IsNAN.cpp
 ELF64-DEBUG-SYMS: 00000014 T _ZN4llvm5IsNANEd
 ELF64-DEBUG-SYMS: 00000000 T _ZN4llvm5IsNANEf
 ELF64-DEBUG-SYMS:          U __isnan
 ELF64-DEBUG-SYMS:          U __isnanf
 
 macho:          U _SomeOtherFunction

llvm/tools/llvm-nm/llvm-nm.cpp

if (SymI->getELFType() == ELF::STT_SECTION) {
     Expected<StringRef> Name = SymI->getName();
     if (!Name) {
       consumeError(Name.takeError());
       return '?';
     }
     return StringSwitch<char>(*Name)
         .StartsWith(".debug", 'N')
         .StartsWith(".note", 'n')
+        .StartsWith(".comment", 'n')
         .Default('?');
   }
 
   return 'n';
 }
 
 static char getSymbolNMTypeChar(COFFObjectFile &Obj, symbol_iterator I) {
   COFFSymbolRef Symb = Obj.getCOFFSymbol(*I);