[libc++] Run the test suite with optimizations by default

In production, libc++ is used with optimizations enabled 99.9% of the
time. Therefore, I think it makes sense to run our tests with optimizations
enabled by default, to be closer to what people actually use in production.
It's still possible to disable optimizations for debugging purposes or
other reasons.

This also removes the pitfall of trying to run the benchmarks but
forgetting to set the optimization level.

Author: ldionne

libcxx/utils/libcxx/test/params.py

@@ -193,7 +193,7 @@ def getSuitableClangTidy(cfg):
         choices=["none", "speed", "size"],
         type=str,
         help="The optimization level to use when compiling the test suite.",
-        default="none",
+        default="speed",
         actions=lambda opt: filter(None, [
             AddCompileFlag(lambda cfg: getSpeedOptimizationFlag(cfg)) if opt == "speed" else None,
             AddCompileFlag(lambda cfg: getSizeOptimizationFlag(cfg)) if opt == "size" else None,