[llvm-objdump] - Fix one more BB.

Should fix the http://lab.llvm.org:8011/builders/polly-amd64-linux/builds/25876/steps/build/logs/stdio:

/home/grosser/buildslave/polly-amd64-linux/llvm.src/tools/llvm-objdump/llvm-objdump.cpp:539:25: error: conditional expression is ambiguous; 'std::string' (aka 'basic_string<char>') can be converted to 'typename std::remove_reference<StringRef>::type' (aka 'llvm::StringRef') and vice versa
      Target = Demangle ? demangle(*SymName) : *SymName;

llvm-svn: 349617