fix potential memory leak in ThreadLocal

fix potential memory leak in ThreadLocal

Summary:
See this LSan abort: https://phabricator.fb.com/P17233565.

Destructor of the object stored in `folly::ThreadLocal` itself may be using
`folly::ThreadLocal` with the same tag, with the current implementation these
objects may escape cleanup happening on thread exit.

Test Plan:
_build/opt/folly/test/thread_local_test --gtest_filter=ThreadLocalPtr.CreateOnThreadExit

Reviewed By: lucian@fb.com

Subscribers: njormrod, folly-diffs@

FB internal diff: D1682698

Tasks: 5596043

Signature: t1:1682698:1416006810:100aaa5c17cecceeea568165d552d9d7907f38d0