[libc++] Make benchmarks dry-run by default on the release branch

As reported in #125510, doing a full run of the benchmarks during
release testing breaks for some of the testers, and it also takes a
long time. The proper fix would be for the release testing process to
call `check-cxx` instead of running lit directly inside libc++'s test
directory: that will also have the benefit of actually running all of
our tests, not only the Lit ones.

However, since that fix may take longer to happen, this patch tries
to reduce the pain of release testers by dry-running benchmarks by
default instead.

Author: ldionne

libcxx/utils/libcxx/test/params.py

@@ -371,7 +371,7 @@ def getSuitableClangTidy(cfg):
         name="enable_benchmarks",
         choices=["no", "run", "dry-run"],
         type=str,
-        default="run",
+        default="dry-run",
         help="Whether to run the benchmarks in the test suite, to only dry-run them or to disable them entirely.",
         actions=lambda mode: [AddFeature(f"enable-benchmarks={mode}")],
     ),