[llvm-nm] Print an explicit "no symbols" message when an object file has no symbols

Summary:
GNU nm (and other nm implementations, such as "go tool nm") prints an explicit "no symbols" message when an object file has no symbols. Currently llvm-nm just doesn't print anything. Adding an explicit "no symbols" message will allow llvm-nm to be used in place of nm: some scripts and build processes use `nm <file> | grep "no symbols"` as a test to see if a file has no symbols. It will also be more familiar to anyone used to nm.

That said, the format implemented here is slightly different, in that it doesn't print the tool name in the message (which IMHO is not useful to include).

Demo:
```
$ for nm in nm bin/llvm-nm ; do echo "nm implementation: $nm"; $nm /tmp/foo{1,2}.o; echo; done
nm implementation: nm

/tmp/foo1.o:
nm: /tmp/foo1.o: no symbols

/tmp/foo2.o:
0000000000000000 T foo2

nm implementation: bin/llvm-nm

/tmp/foo1.o:
no symbols

/tmp/foo2.o:
0000000000000000 T foo2
```

Reviewers: MaskRay

Reviewed By: MaskRay

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D52810

llvm-svn: 343742