[libcxx] Enable C++17 for the benchmarks.

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.

Differential Revision: https://reviews.llvm.org/D75955