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

[kubamracek]

Summary

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);

include/sanitizer/tsan_interface.h

 // profiling code.
 // These annotations must be called only from within
 // __tsan_mutex_pre/post_lock, __tsan_mutex_pre/post_unlock,
 // __tsan_mutex_pre/post_signal regions.
 // Supported flags: none.
 void __tsan_mutex_pre_divert(void *addr, unsigned flags);
 void __tsan_mutex_post_divert(void *addr, unsigned flags);
 
+// External race detection API.
+// Can be used by non-instrumented libraries to detect when their objects are
+// being used in an unsafe manner.
+//   - __tsan_external_read/__tsan_external_write annotates the logical reads
+//       and writes of the object at the specified address. 'caller_pc' should
+//       be the PC of the library user, which the library can obtain with e.g.
+//       `__builtin_return_address(0)`.
+//   - __tsan_external_register_tag registers a 'tag' with the specified name,
+//       which is later used in read/write annotations to denote the object type
+//   - __tsan_external_assign_tag can optionally mark a heap object with a tag
+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);
+
 #ifdef __cplusplus
 }  // extern "C"
 #endif
 
 #endif  // SANITIZER_TSAN_INTERFACE_H