Document how to find the path of executables in the test suite

Author: ldionne

libcxx/docs/TestingLibcxx.rst

@@ -448,7 +448,16 @@ For more information about using the Google Benchmark library, see the
 The benchmarks are located under ``libcxx/test/benchmarks``. Running a benchmark
 works in the same way as running a test. Both the benchmarks and the tests share
 the same configuration, so make sure to enable the relevant optimization level
-when running the benchmarks.
+when running the benchmarks. For example,
+
+.. code-block:: bash
+
+  $ libcxx/utils/libcxx-lit <build> -sv libcxx/test/benchmarks/string.bench.cpp --param optimization=speed
+
+If you want to see where a benchmark is located (e.g. you want to store the executable
+for subsequent analysis), you can print that information by passing ``--show-all`` to
+``lit``. That will print the command-lines being executed, which includes the location
+of the executable created for that benchmark.
 
 Note that benchmarks are only dry-run when run via the ``check-cxx`` target since
 we only want to make sure they don't rot. Do not rely on the results of benchmarks