[asan] Turn LSan-related #if’s into regular if’s in ASan initializer

[kubamracek]

Summary

Removing some preprocessor #if’s in favor of regular if’s. However, we need to declare empty stub functions to avoid linker errors.

[kcc]

LGTM (assuming everything continues to build)
Thank you!

lib/asan/asan_rtl.cc

AsanThread *main_thread = AsanThread::Create(
       /* stack */ nullptr, /* detached */ true);
   CHECK_EQ(0, main_thread->tid());
   SetCurrentThread(main_thread);
   main_thread->ThreadStart(internal_getpid(),
                            /* signal_thread_is_registered */ nullptr);
   force_interface_symbols();  // no-op.
   SanitizerInitializeUnwinder();
 
-#if CAN_SANITIZE_LEAKS
+  if (CAN_SANITIZE_LEAKS) {
     __lsan::InitCommonLsan();
     if (common_flags()->detect_leaks && common_flags()->leak_check_at_exit) {
       Atexit(__lsan::DoLeakCheck);
     }
-#endif  // CAN_SANITIZE_LEAKS
+  }
 
 #if CAN_SANITIZE_UB
   __ubsan::InitAsPlugin();
 #endif
 
   InitializeSuppressions();
 
-  {
-#if CAN_SANITIZE_LEAKS
+  if (CAN_SANITIZE_LEAKS) {
     // LateInitialize() calls dlsym, which can allocate an error string buffer
     // in the TLS.  Let's ignore the allocation to avoid reporting a leak.
     __lsan::ScopedInterceptorDisabler disabler;
-#endif
+    Symbolizer::LateInitialize();
+  } else {
     Symbolizer::LateInitialize();
   }
 
   VReport(1, "AddressSanitizer Init done\n");
 }
 
 // Initialize as requested from some part of ASan runtime library (interceptors,
 // allocator, etc).

lib/lsan/lsan_common.cc

 uptr LeakReport::UnsuppressedLeakCount() {
   uptr result = 0;
   for (uptr i = 0; i < leaks_.size(); i++)
     if (!leaks_[i].is_suppressed) result++;
   return result;
 }
 
 } // namespace __lsan
+#else // CAN_SANITIZE_LEAKS
+namespace __lsan {
+void InitCommonLsan() { }
+void DoLeakCheck() { }
+void DisableInThisThread() { }
+void EnableInThisThread() { }
+}
 #endif // CAN_SANITIZE_LEAKS
 
 using namespace __lsan;  // NOLINT
 
 extern "C" {
 SANITIZER_INTERFACE_ATTRIBUTE
 void __lsan_ignore_object(const void *p) {
 #if CAN_SANITIZE_LEAKS