Fix clang-tidy performance warnings.

* Use const reference parameter type to avoid unnecessary copy.

Bug: 30407689
Change-Id: Iefab05c077367f272abf545036b853e8a295c8cd
Test: build with WITH_TIDY=1

tools/atree/atree.cpp

@@ -92,7 +92,7 @@ debug_printf(const char* format, ...)
 
 // Escape the filename so that it can be added to the makefile properly.
 static string
-escape_filename(const string name)
+escape_filename(const string& name)
 {
     ostringstream new_name;
     for (string::const_iterator iter = name.begin(); iter != name.end(); ++iter)