[sanitizer] Don't reuse the main thread in ThreadRegistry

There is a hard-to-reproduce crash happening on OS X that involves terminating the main thread (dispatch_main does that, see discussion at http://reviews.llvm.org/D18496) and later reusing the main thread's ThreadContext. This patch disables reuse of the main thread. I believe this problem exists only on OS X, because on other systems the main thread cannot be terminated without exiting the process.

Differential Revision: http://reviews.llvm.org/D19722

llvm-svn: 268238