[tools] [llvm-nm] Default to reading from stdin not a.out

Summary: This moves away from defaulting to a.out and uses stdin only if stdin has a file redirected to it. This has been discussed on the llvm-dev mailing list [[ https://lists.llvm.org/pipermail/llvm-dev/2019-July/133642.html | here ]].

Reviewers: jhenderson, rupprecht, MaskRay, chrisjackson

Reviewed By: jhenderson, MaskRay

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64290

llvm-svn: 365889