[sanitizer] Intercept setpwent/endpwent.

It's hard to write a reliable test for this code because they
work with unpredictable memory locations. But this change should
fix current failures in getpwent() tests on the sanitizer bots.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@205002 91177308-0d34-0410-b5e6-96231b3b80d8