Usage of atomic_thread_fence in shared_ptr.h can cause false-positives w/ TSAN

[romasandu-gaijin]

Thread sanitizer does not work with atomic_thread_fences and unlikely ever will. Every usage of atomic_thread_fence must be guarded by an #if !TSAN or a similar construction for correctness of synchronization to be verifiable via TSAN.

This also breaks TSAN compilation with latest GCC, as it now warns about usage of fences when TSAN is enabled (and rightly so!)

[grojo-ea]

Hello, thanks for your report, we have an internal fix which is slightly different than what you suggest in your PR. We prefer not to branch behaviour when TSAN is enabled vs when it's not so we've done some TSAN markup under the hood to make it happy.

Let us know if you still have this issue as of 0b8a259.