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.