Eric thought that Darwin was right to use -1 consistently rather than
leaving this undefined, and despite the sentence in the standard that
seems to require it, I'll cede the point and assume its a bug in the
wording. Other parts of POSIX regularly allow for things to be -1
instead of undefined, this should too. Makes things more consistent too.

This should have to real impact for folks though.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171574 91177308-0d34-0410-b5e6-96231b3b80d8