[libc++][cmake] Improves benchmark build.

[Mordante]

Summary

The CMake dependencies don't properly list the libc++ headers. When a
libc++ header is modified the affected benchmarks aren't rebuild. This
makes testing benchmarks tricky and may cause accidentally not using the
latest modifications during testing. This change causes CMake to
determine the proper dependencies.

This shouldn't affect the CI build.

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)
-target_include_directories(cxx-benchmarks-flags INTERFACE "${BENCHMARK_LIBCXX_INSTALL}/include"
+target_compile_options(    cxx-benchmarks-flags INTERFACE -O2 -fsized-deallocation -nostdinc++)
+target_include_directories(cxx-benchmarks-flags INTERFACE "${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")
 if (LIBCXX_BENCHMARK_NATIVE_STDLIB STREQUAL "libstdc++")
   find_library(LIBSTDCXX_FILESYSTEM_TEST stdc++fs