Implemented the code to make llvm-nm’s -g option work.

Implemented the code to make llvm-nm’s -g option work.

While llvm-nm parses the -g option and has help that describes it as:

  -extern-only            - Show only external symbols

There is no code in the program to use the boolean valve it sets from the
command line.

rdar://23261095

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@251718 91177308-0d34-0410-b5e6-96231b3b80d8