[tsan] Publish the TSan external API in tsan_interface.h

Let's make the TSan external API available and commented in the public header:

    void *__tsan_external_register_tag(const char *object_type);
    void __tsan_external_assign_tag(void *addr, void *tag);
    void __tsan_external_read(void *addr, void *caller_pc, void *tag);
    void __tsan_external_write(void *addr, void *caller_pc, void *tag);

Differential Revision: https://reviews.llvm.org/D32358

llvm-svn: 301003