prevent asan failures in SingletonThreadLocal

Summary:
I have been encountering

```
AddressSanitizer:DEADLYSIGNAL
=================================================================
==937752==ERROR: AddressSanitizer: SEGV on unknown address 0x24a12000e4f18 (pc 0x7f3902a6e696 bp 0x7f37bc5e4380 sp 0x7f37bc5e42a0 T107)
==937752==The signal is caused by a READ memory access.
SCARINESS: 20 (wild-addr-read)

    #0 0x7f3902a6e695 in folly::ThreadLocalPtr<folly::SingletonThreadLocal<folly::hazptr_tc<std::atomic>, void, folly::detail::DefaultMake<folly::hazptr_tc<std::atomic> >, void>::Wrapper, void, void>::get() const folly/ThreadLocal.h:160
    facebook#1 0x7f3902a6d600 in get folly/ThreadLocal.h:69
    facebook#2 0x7f3902a6d600 in folly::ThreadLocal<folly::SingletonThreadLocal<folly::hazptr_tc<std::atomic>, void, folly::detail::DefaultMake<folly::hazptr_tc<std::atomic> >, void>::Wrapper, void, void>::operator*() const folly/ThreadLocal.h:78
    facebook#3 0x7f3902a6ca1b in folly::SingletonThreadLocal<folly::hazptr_tc<std::atomic>, void, folly::detail::DefaultMake<folly::hazptr_tc<std::atomic> >, void>::getWrapper() folly/SingletonThreadLocal.h:149
    facebook#4 0x7f3902a6cbe5 in folly::SingletonThreadLocal<folly::hazptr_tc<std::atomic>, void, folly::detail::DefaultMake<folly::hazptr_tc<std::atomic> >, void>::LocalLifetime::~LocalLifetime() folly/SingletonThreadLocal.h:121
    facebook#5 0x7f38f04d9fd5 in (anonymous namespace)::run(void*) /data/users/kirkshoop/gcc/trunk/libstdc++-v3/libsupc++/atexit_thread.cc:75:16
    facebook#6 0x7f38ef9ee551 in __nptl_deallocate_tsd.part.8 /home/engshare/third-party2/glibc/2.26/src/glibc-2.26/nptl/pthread_create.c:301:8
    facebook#7 0x7f38ef9ef7c8 in __nptl_deallocate_tsd /home/engshare/third-party2/glibc/2.26/src/glibc-2.26/sysdeps/nptl/futex-internal.h:200:3
    facebook#8 0x7f38ef9ef7c8 in start_thread /home/engshare/third-party2/glibc/2.26/src/glibc-2.26/nptl/pthread_create.c:475:3
```

I tried using a unique Tag for the SingletonThreadLocal for debugging purposes and this seemed to fix the issue.

Reviewed By: magedm

Differential Revision: D19356179

fbshipit-source-id: a18c6b02f0a40e0509753075614eeff71d859bf7

folly/synchronization/HazptrThrLocal.h

@@ -149,10 +149,11 @@ class hazptr_tc {
   }
 }; // hazptr_tc
 
+struct hazptr_tc_tls_tag {};
 /** hazptr_tc_tls */
 template <template <typename> class Atom>
 FOLLY_ALWAYS_INLINE hazptr_tc<Atom>& hazptr_tc_tls() {
-  return folly::SingletonThreadLocal<hazptr_tc<Atom>, void>::get();
+  return folly::SingletonThreadLocal<hazptr_tc<Atom>, hazptr_tc_tls_tag>::get();
 }
 
 /**