[libc++] Require C++20 to build the benchmarks.

[Mordante]

Summary

Some work-in-progress patches for the format header contain benchmarks.
The format header requires C++20 to build. This is a preparation to make
it easy to add these benchmarks.

[ldionne]

This LGTM, let's see if other build bots not covered by our CI complain.

If they do, then we need to migrate them to our CI :-).

[ldionne]

libcxx/docs/ReleaseNotes.rst

41

I would move this to API changes, though. It's not really a feature. No need to update the review.

[Mordante]

In D96057#2551513, @ldionne wrote:

This LGTM, let's see if other build bots not covered by our CI complain.

If they do, then we need to migrate them to our CI :-).

If it's that easy ;-)

[Mordante]

The sanitizer-ppc64be-linux build node uses gcc 7.3.1 and fails http://lab.llvm.org:8011/#/builders/18/builds/749

libcxx/benchmarks/CMakeLists.txt

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 "${BENCHMARK_TEST_LIBCXX_COMPILE_FLAGS}"
           LINK_FLAGS "${BENCHMARK_TEST_LIBCXX_LINK_FLAGS}"
-          CXX_STANDARD 17
+          CXX_STANDARD 20
           CXX_STANDARD_REQUIRED YES
           CXX_EXTENSIONS NO)
   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()
if (LIBCXX_BENCHMARK_NATIVE_STDLIB)
     add_dependencies(cxx-benchmarks ${native_target})
     set_target_properties(${native_target}
       PROPERTIES
           OUTPUT_NAME "${name}.native.out"
           RUNTIME_OUTPUT_DIRECTORY "${BENCHMARK_OUTPUT_DIR}"
           INCLUDE_DIRECTORIES ""
           COMPILE_FLAGS "${BENCHMARK_TEST_NATIVE_COMPILE_FLAGS}"
           LINK_FLAGS "${BENCHMARK_TEST_NATIVE_LINK_FLAGS}"
-          CXX_STANDARD 17
+          CXX_STANDARD 20
           CXX_STANDARD_REQUIRED YES
           CXX_EXTENSIONS NO)
   endif()
 endfunction()
 
 
 #==============================================================================
 # Register Benchmark tests

libcxx/docs/ReleaseNotes.rst

 see the `releases page <https://llvm.org/releases/>`_.
 
 What's New in Libc++ 13.0.0?
 ============================
 
 New Features
 ------------
 
 - ...
 
 API Changes
 -----------
 
-- ...
+- Starting with libc++13 a C++20 capable compiler is required to build the
+  libc++ benchmarks.