Use the information provided by getFlags to unify some code in llvm-nm.

It is not clear how much we should try to expose in getFlags. For example,
should there be a SF_Object and a SF_Text?

But for information that is already being exposed, we may as well use it in
llvm-nm.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200820 91177308-0d34-0410-b5e6-96231b3b80d8