[libc++] Save benchmark results in a json file (#119761)

When running a benchmark, also save the benchmark results in a JSON
file. That is cheap to do and useful to compare benchmark results
between different runs.

Author: ldionne

libcxx/utils/libcxx/test/format.py

@@ -348,7 +348,7 @@ def execute(self, test, litConfig):
                 "%dbg(COMPILED WITH) %{cxx} %s %{flags} %{compile_flags} %{benchmark_flags} %{link_flags} -o %t.exe",
             ]
             if "enable-benchmarks=run" in test.config.available_features:
-                steps += ["%dbg(EXECUTED AS) %{exec} %t.exe"]
+                steps += ["%dbg(EXECUTED AS) %{exec} %t.exe --benchmark_out=%T/benchmark-result.json --benchmark_out_format=json"]
             return self._executeShTest(test, litConfig, steps)
         else:
             return lit.Test.Result(