Change the default input for llvm-nm to be a.out instead of standard input

Change the default input for llvm-nm to be a.out instead of standard input
to match llvm-size and other UNIX systems for their nm(1).

Tweak test cases that used llvm-nm with standard input to add a "-" to
indicate that and add a test case to check the default of a.out for llvm-nm.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211529 91177308-0d34-0410-b5e6-96231b3b80d8