Move the uglier parts of deciding not to emit a

Move the uglier parts of deciding not to emit a
UsedDirective for some symbols in llvm.used into
Darwin-specific code.  I've decided LessPrivateGlobal
is potentially a useful abstraction and left it in
the target-independent area, with improved comment.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@56024 91177308-0d34-0410-b5e6-96231b3b80d8