[libc++] Don't force -O2 when building the benchmarks

[ldionne]

Summary

The optimization level used when building the benchmarks should
match the optimization level of the current build. Otherwise, we
can end up mixing an -O3 or -O0 optimized dylib with benchmarks
built with -O2, which is really misleading.

[Mordante]

LGTM, thanks!

libcxx/benchmarks/CMakeLists.txt

 #==============================================================================
 add_custom_target(cxx-benchmarks)
 set(BENCHMARK_OUTPUT_DIR ${CMAKE_CURRENT_BINARY_DIR})
 set(BENCHMARK_LIBCXX_INSTALL ${CMAKE_CURRENT_BINARY_DIR}/benchmark-libcxx)
 set(BENCHMARK_NATIVE_INSTALL ${CMAKE_CURRENT_BINARY_DIR}/benchmark-native)
 
 add_library(               cxx-benchmarks-flags INTERFACE)
 target_compile_features(   cxx-benchmarks-flags INTERFACE cxx_std_20)
-target_compile_options(    cxx-benchmarks-flags INTERFACE -O2 -fsized-deallocation -nostdinc++)
+target_compile_options(    cxx-benchmarks-flags INTERFACE -fsized-deallocation -nostdinc++)
 target_include_directories(cxx-benchmarks-flags INTERFACE "${LIBCXX_GENERATED_INCLUDE_DIR}"
                                                 INTERFACE "${BENCHMARK_LIBCXX_INSTALL}/include"
                                                 INTERFACE "${LIBCXX_SOURCE_DIR}/test/support")
 
 add_library(           cxx-benchmarks-flags-native INTERFACE)
 target_link_libraries( cxx-benchmarks-flags-native INTERFACE cxx-benchmarks-flags)
 target_compile_options(cxx-benchmarks-flags-native INTERFACE ${BENCHMARK_NATIVE_TARGET_FLAGS})
 target_link_options(   cxx-benchmarks-flags-native INTERFACE ${BENCHMARK_NATIVE_TARGET_FLAGS} "-L${BENCHMARK_NATIVE_INSTALL}/lib")