[sanitizer] Don't unpoison buffer in getpw/getgr functions

Summary:
Buffer should be referenced by results so used parts will be unpoisoned with unpoison_group and unpoison_passwd.

This fixes TSAN performance issue made us to disable this interceptors.

Reviewers: eugenis, dvyukov

Subscribers: srhines, kubamracek, krytarowski, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D57731

llvm-svn: 353351