Warn when no external symbolizers are found.

Author: ndrewh

compiler-rt/lib/sanitizer_common/sanitizer_symbolizer_posix_libcdep.cpp

@@ -505,6 +505,13 @@ static void ChooseSymbolizerTools(IntrusiveList<SymbolizerTool> *list,
   }
 
 #  if SANITIZER_APPLE
+  if (list.size() == 0) {
+    Report(
+        "WARN: No external symbolizers found. Symbols will be missing or "
+        "unreliable.\n");
+    Report(
+        "HINT: Is PATH set? Does sandbox allow file-read of /usr/bin/atos?\n");
+  }
   VReport(2, "Using dladdr symbolizer.\n");
   list->push_back(new (*allocator) DlAddrSymbolizer());
 #  endif  // SANITIZER_APPLE