Update llvm-nm -s to use a multi-var option

Summary:
Previously llvm-nm relied on a positional parameter to read two values
into the SegSect list. This worked, but required the "-s" paramater and
its arguments to be the last elements on the command-line. 

The CommandLine library now supports mutli-var parameters, so it can
naturally deal with "-s" expecting two arguments, and now the input file
can appear anywhere (within reason) in the command line invocation. E.g.

    llvm-nm -s __TEXT __text /bin/ls
    llvm-nm /bin/ls -s __TEXT __text

rdar://27284011

Reviewers: lhames, pete

Reviewed By: pete

Subscribers: rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62070

llvm-svn: 361091