[libcxx] Enable C++17 for the benchmarks

[Mordante]

Summary

The benchmarks are intended to be build with C++17 but the CMAKE_CXX_STANDARD in the LLVM forces the build to use C++14 by default. This fixes the issue by setting the CXX_STANDARD property of the benchmark targets.

The CMake documentation is not clear whether this will use the C++1z fallback for older compilers. So this may break the benchmarks if somebody uses the benchmarks with pre C++17 compilers with the C++1z fallback.

This was discovered while investigating the build issue of D62778.

[Mordante]

Noticed I didn't stage my last hunk. Don't use BENCHMARK_DIALECT_FLAG which no longer is set.

[ldionne]

LGTM since CXX_STANDARD 17 will decay to an older standard if the compiler doesn't support -std=c++17. It seems to be the behavior we want (although I would argue that setting CXX_STANDARD_REQUIRED would also make sense).

[Mordante]

CXX_STANDARD_REQUIRED is already set by the main LLVM CMake script.

[Mordante]

Seems it requires CMake 3.8 to use CXX_STANDARD 17, so reverted the commit for now.

libcxx/benchmarks/CMakeLists.txt

 #==============================================================================
 # Benchmark tests configuration
 #==============================================================================
 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)
 
-check_flag_supported("-std=c++17")
-mangle_name("LIBCXX_SUPPORTS_STD_EQ_c++17_FLAG" BENCHMARK_SUPPORTS_STD_CXX17_FLAG)
-if (${BENCHMARK_SUPPORTS_STD_CXX17_FLAG})
-  set(BENCHMARK_DIALECT_FLAG "-std=c++17")
-else()
-  # If the compiler doesn't support -std=c++17, attempt to fall back to -std=c++1z while still
-  # requiring C++17 language features.
-  set(BENCHMARK_DIALECT_FLAG "-std=c++1z")
-endif()
-
 set(BENCHMARK_TEST_COMPILE_FLAGS
-    ${BENCHMARK_DIALECT_FLAG} -O2
+    -O2
     -fsized-deallocation
     -I${BENCHMARK_LIBCXX_INSTALL}/include
     -I${LIBCXX_SOURCE_DIR}/test/support
 )
 set(BENCHMARK_TEST_LIBCXX_COMPILE_FLAGS
     -nostdinc++
     -isystem ${LIBCXX_SOURCE_DIR}/include
     ${BENCHMARK_TEST_COMPILE_FLAGS}
function(add_benchmark_test name source_file)
   if (LLVM_USE_SANITIZER)
     target_link_libraries(${libcxx_target} PRIVATE -ldl)
   endif()
   set_target_properties(${libcxx_target}
     PROPERTIES
           OUTPUT_NAME "${name}.libcxx.out"
           RUNTIME_OUTPUT_DIRECTORY "${BENCHMARK_OUTPUT_DIR}"
           COMPILE_FLAGS "${BENCHMARK_TEST_LIBCXX_COMPILE_FLAGS}"
+          CXX_STANDARD 17
           LINK_FLAGS "${BENCHMARK_TEST_LIBCXX_LINK_FLAGS}")
   cxx_link_system_libraries(${libcxx_target})
   if (LIBCXX_BENCHMARK_NATIVE_STDLIB)
     if (LIBCXX_BENCHMARK_NATIVE_STDLIB STREQUAL "libstdc++" AND NOT DEFINED LIBSTDCXX_FILESYSTEM_LIB
         AND "${name}" STREQUAL "filesystem")
       return()
     endif()
     set(native_target ${name}_native)