[analyzer] Model strsep(), particularly that it returns its input.

This handles the false positive leak warning in PR15374, and also serves
as a basic model for the strsep() function.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@180069 91177308-0d34-0410-b5e6-96231b3b80d8