Use pthreads to manage thread-local storage on darwin for leak sanitizer

Summary:
__thread is supported on Darwin, but is implemented dynamically via
function calls to __tls_get_addr. This causes two issues when combined
with leak sanitizer, due to malloc() interception.

- The dynamic loader calls malloc during the process of loading
  the sanitizer dylib, while swapping a placeholder tlv_boostrap
  function for __tls_get_addr. This will cause tlv_bootstrap to
  be called in DisabledInThisThread() via the asan allocator.

- The first time __tls_get_addr is called, it allocates memory
  for the thread-local object, during which it calls malloc(). This
  call will be intercepted, leading to an infinite loop in the asan
  allocator, in which the allocator calls DisabledInThisThread,
  which calls tls_get_addr, which calls into the allocator again.

Reviewers: kcc, glider, kubamracek

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29786

llvm-svn: 294994