[sanitizer] Don't unpoison buffer in getpw/getgr functions
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
Differential Revision: https://reviews.llvm.org/D57731